The lead-up to Hearth's Warming always had a different energy in Canterlot than Ponyville. Attuned as she was to ponies' moods, Pinkie couldn't help but pick up on the subtle shifts in the holiday cheer, how ponies paid less attention to the joy of their neighbor than how much more they were spending on celebrations. It wasn't always the case, but it definitely got more frequent as she approached the palace.

She made a few mental notes to revisit the issue. She was Minister of Laughter now, after all. This sort of thing was her responsibility... and arguably Rarity's. Either way, she'd have to look into it when Twilight hadn't sent an emergency "Come to the palace ASAPP*" notice.
* As Soon As Pinkiely Possible, which by all rights meant she should have been there by now... Uh oh.

Shaking her head to get her thoughts out of her mental footnotes, Pinkie shifted to full gallop. She barely acknowledged the guards flinching in her wake, slid under the last set of crossed spears like they were a Limbo bar, and left the summons fluttering to the ground in her wake so they wouldn't feel bad. Or arrest her.

Again.

To Pinkie's relief, Twilight was still on the throne instead of wearing a groove into the marble, even if the reigning princess's fidgeting wings and tail betrayed how much the delay was bothering her. "Pinkie! Thank goodness you're here. Raven, I trust you can see to rest of the holiday staff roster without me?"

Raven rolled her eyes, but the same way Limestone did when she didn't want to admit Pinkie was being funny. "With respect, Your Highness, you insisted on helping me with it in the first place." She dipped into a shallow bow and made her way out of the throne room. "I'll leave you two to the next pressing matter. Happy Hearth's Warming, Minister Pie."

"Happy Hearth's Warming!" said Pinkie, because some things were important. She pronked up to the foot of the throne and rubbed her forehooves together. "Okay, Twilight, what's the problem?"

Twilight took a moment before answering, her eyes looking into the distance as her mind withdrew to reshelve all her thoughts. After a few seconds, she said, “How familiar are you with the chiroptera?”

It took Pinkie a moment to place the term. “Oh! You mean bat ponies?”

That got a facehoof. “Please don’t call them that to their faces.”

Pinkie shrugged. “The Mangoes didn’t mind.”

Twilight furrowed her brow. “The who?”

“They were looking into moving into Ponyville a bit ago, but above-ground houses don’t really work for them and digging out a new cave would take months and months if we don’t want to collapse Maud’s grotto or any of the tunnels." Pinkie considered Twilight's current mental state and added, "Which we don’t.”

A spiral-bound notepad flashed into being next to Twilight, a floating pencil scribbling something in it without her looking. “Something to think about for later," she said, making it vanish. "But I called you here today because of their holidays.”

“Ooh!” Pinkie whipped her day planner out of her tail and flipped ahead a few pages. “Lay ‘em on me. The Mangoes didn’t stay in town long enough for me to ask.”

“The important one is…" Twilight opened her mouth, paused, and shook her head. "Well, I can’t pronounce the ultrasonic phonemes and you can’t hear them, but it translates as 'The Festival of Stars.' I've attended the first night of the festivities since taking the throne, but Hearth’s Warming is happening on the same day this year.”

"But Hearth's Warming is on the same day every year." Pinkie flipped to that page in her planner, just to be sure.

"It is. The chiroptera went back to their traditional lunar calendar after Nightmare Moon’s banishment."

Pinkie tilted her head in thought. “Wait, where’s the calendar on Luna? Is it some of the dapples where you can’t tell if they’re part of her cutie mark or not?”

Twilight smiled at that. Good. She'd gotten a bit more comfortable on the throne, but she still needed all the smiles she could get. “A calendar based on the moon rather than the sun. The two aren’t in perfect sync, so dates on one calendar drift if you’re using the other." She started speaking faster and faster, taking flight so she could pace in midair. It was that bad. "According to records from the Late Paleopony Period, that’s how the sun and moon behaved before the Breaking of the Sky. Unicorn astrological circles tried their best to keep that pattern going, but the waning number of available casters just before Celestia took the sun made the drift even worse, to say nothing of Discord’s influence. Celestia and Luna could have theoretically synced up the cycles, but they chose to maintain them for reasons they've kept to themselves. Because obviously they wouldn’t tell such unimportant details to the mare who’s doing their job for them.” That prompted a long sigh. “Sorry. Lecture’s over. How much of that did you get?”

“Scheduling conflict,” said Pinkie.

After a brief pause, Twilight landed next to her and nodded. “A more succinct explanation, yes. It’s happened before, of course, but either the chiroptera were shunning Celestia or, in the most recent case, Luna was there to celebrate the festival with them.”

“And shun Celestia,” said the mare with two older sisters.

“And shun Celestia,” agreed the one with an older brother. “But we’re back to a one-princess model, I haven’t done anything to offend the chiroptera yet, and I’d really rather not start now. So what do I do?”

Pinkie tapped her chin. She had something approaching a plan. A bunch of plans, actually. Best to work through the most complete ideas first. “I’m guessing something like that time Rarity tried to be at your birthday and a Canterlot garden party at the same time won’t work.”

Twilight shook her head. “It was a bad idea then and a worse one now. If it weren’t for Fancy Pants, Applejack might still be banned from the Royal Conservatory.”

“Do you have to be at both parties?”

“Offending the chiroptera could undo all of the goodwill we’ve built up since Luna’s return. Offending the Canterlot upper crust absolutely will make my daily life a headache for years to come." Twilight spread her wings, giving them a rueful look. "I am the princess, after all.”

Pinkie pulled her into a hug. “Yeah, and we’re your assistant princess minister council friends! Could you send one of us?”

“In theory, yes," Twilight said after squeezing back, "but I’m not sure how the chiroptera would respond to a representative.”

That got a gasp as inspiration struck. “Ooh, you could re-bat Fluttershy! I bet they’d love that!”

Twilight rolled her eyes. "Suuuure," she drawled. "Use unicorn magic to turn a feathered pegasus into a crude facsimile of a chiropteron that instinctually ravages earth pony farms. Let me know if you think of a way to offend kirin while we're at it.”

That brought up Pinkie short. “Wow. It’s been a long time since I saw you this sarcastic.”

“Letting my various complexes run wild isn’t a viable coping mechanism anymore, not when I’m in the public eye.”

Pinkie frowned. “So that night at Trivia Trot—”

“We’ll talk about that after we get this sorted out.” Twilight summoned her notebook again, the pencil scribbling as she performed the appropriate gestures. “Pinkie Promise.”

“Okay…" Pinkie gasped. "Wait!”

“What?”

“Your whole problem is that there’s only one of you and two parties you need to go to, right?” said Pinkie, pointing and spreading her hooves.

Twilight nodded, frowning. “As I’ve explained, yes.”

Pinkie brought her hooves together and beamed. “So, we just bring the two parties together! Stars’ Warming Festeve!”

The frown persisted. Stubborn little thing. “Isn’t that just Rarity’s idea again?”

“No, silly, it’s Rainbow Dash’s idea! Show those chiroptoponies how we party Ponyville-style!" After a moment, Pinkie shrugged. "Or Canterlot-style, but hey, good excuse to liven up a stuffy old event.”

Twilight shook her head. “There’s no way that could work.”

“Why not?”

"It... Hmm." The frown finally started to shift. Into a different, more uncertain frown, but progress was progress. “Still, the logistics—”

Pinkie gave a dismissive nicker. “Oh, I never said it would be simple. But you’re the Princess of Friendship! Bringing creatures together is your whole thing.”

Finally, Twilight allowed herself another smile, and just the teensiest bit of relaxation. “This is exactly why I called you in for this. You won’t mind helping me plan this mess?”

“Mind?" Pinkie grinned. "I’d mind if you told me not to help!”

That was hardly the end of the delegation. Indeed, Twilight and Pinkie then proceeded to delegate the delegation.

Pinkie leveled a hard stare at her most trusted assistant. “I trust you understand why I’ve called you here today.”

“Yes, Ma’am.”

The acoustics of the throne room helped ensure her voice had all the booming intensity it needed. “You are the best pony we have available. You may well be the only pony available for the task before you.”

That got a salute. “Ma’am, I shall live up to your expectations if not exceed them!”

Pinkie giggled and kissed Cheese Sandwich on the tip of his muzzle, her husband's own chuckles making his saluting hoof shake. “That’s what I’m hoping for.”

Lemon Hearts cleared her throat. “This is sweet and all, Twilight, but why did you call me in again?”

“We need to approach this from all angles if it’s going to work," Twilight said as she paced. At least she was on the ground this time. "You have years of experience with palace events; you know what the upper crust will expect and how they enjoy themselves. But you’re not so steeped in tradition that you can’t adapt to new circumstances.”

“And I’m your friend from school and more willing to go along with you on this than the rest of the event staff,” Lemon said with a smirk.

Twilight matched the expression. “Your words, not mine. Meanwhile, Cheese has partied on every conceivable venue in Equestria and a good number out of it.”

“Including the Forbidden Jungles!" Cheese pulled out a pith helmet, still studded with a few exotically fletched arrows. "This won’t be my first Festival of Stars, Your Highness.”

Lemon stared at him for a few moments. “Why were you in the Forbidden Jungles?”

He shrugged. “Hey, Ahuizotl only turns eight hundred once.”

That prompted a baffled turn to Twilight, who patted the unicorn on the withers. “Forget it, Lemon. They’re party ponies.”

“You still want me to work with him.”

“Cheese, like Pinkie, is as brilliant as he is eccentric." Twilight gave a regal smile, looking from one assistant to the other. "I have every faith that the two of you will be instrumental in planning an event that will delight all attendees.”

Lemon attempted a smile of her own. Pinkie managed to hold back a wince. “Uh… sure. How hard could it—?”

Twilight teleported the few feet needed to shove a hoof in Lemon's muzzle. “For your own sake, I have to insist that you not finish that sentence." After a moment, her face scrunched up. "Also, on a scale from one to Celestia, how much pressure am I unintentionally putting on you?”

“Um…" Lemon tapped a hoof against the floor as she thought. "A few Cadences?”

That got a thoughtful nod. “Not as bad as I thought, but still more than I’d like. Something to work on in the future.”

“I'm not complaining,” said Cheese, "but why do you need us to lead the charge on setup?"

“Pinkie and I need to recruit the last member of your team," said Twilight. "And for that, we have to clear the idea with the foremost figure in the local chiropteron community.”

Lemon blinked. “We have a local chiropteron community?”

Twilight gave an understanding nod. “I was as surprised as you when I met him at my accession. Turns out the old gem mines weren’t as abandoned as we thought…”

For years after the wedding, Twilight had never expected to return to the crystal mines beneath Canterlot. But her whole reign thus far had been an unexpected learning experience. Learning of the other layer of her hometown was par for the course compared to the paranoid scions of ancient heroes trying to tear the city off of the mountain entirely.

(She almost wished it were the Knights of Cunabula again. Problems she could solve with coherent beams of Harmony were so much simpler than what she dealt with these days.)

This wasn't her first trip into the undercity, but it was still tricky business remembering the right way. There were other routes, but being princess meant that she couldn't just casually trot into town through the back streets, especially not on such short notice. Plus, she had to keep an eye on Pinkie.

Twilight glanced behind herself, sighed, and said, "Please don't lick the crystals."

Pinkie scoffed. "Shows what you know. Depleted thaumium tastes like snozzberries!" She gave one of the crystalline stalagmites one last taste before catching up, smacking her lips along the way.

"Look, it's only an hour or two after sunset. It'll be early for everypony down there and I don't want to impose any more than I already am."

"Hey, when you have big, princessy horseshoes to fill, sometimes you have to take big, messy steps." Pinkie glanced down at Twilight's hooves. She was in full regalia, hipposandals included. There was no getting away from it these days. "Not that you've grown into the size Celestia ones, but still."

Twilight giggled at that. "Can you imagine?" She stopped for a moment and found that she could. She swore she heard her spine creak ominously. That or an old mining support strut somewhere. "Never mind, let's just get this over with."

As they descended, the caverns got a bit smoother, the path a bit easier. Soon enough, they were trotting on something like tile, or at least a smoothed down layer of cavern floor. Phosphorescent fungi grew in patches too big to be coincidence, supplemented here and there by dim-glowing gems.

In time, they arrived at wrought iron gates, the tops decorated with little crescent moons. A chiropteron guard stood on each side, one at attention, the other halfway through a squeaky yawn.

Twilight nodded. "Gentlecolts."

The yawner jumped, belted out a completely inaudible scream, and delivered a salute that might have left him concussed if it weren't for his helmet. The other guard glanced at him, gave a smirk Twilight only spotted after years of knowing Shining's friends in the Guard, and nodded back. "Your Highness. Pinkie."

"Hi, Duskwing!"

Twilight didn't even blink. "We won't be here long. Just need to see the archbishop."

"O-of course!" said the less composed guard. To his credit, he worked in perfect synchrony with his partner to unlatch and open the gates, along with the official greeting: "Welcome to Canterburied!"

The crystals and glowing mushrooms continued within the city, sometimes arranged in little gardens in front of houses. A pitched slope led deep into the heart of the mountain, creating a space big enough that support pillars towered in the darkness, visible by the light of apartments within.

Twilight kept to the edge. "We're staying in Old Town. The cathedral isn't far."

Pinkie was having a wonderful time going by how she was eagerly looking at everything. "This is amazing! I thought you might have to light up the place yourself."

"Even chiroptera need some light to work with. And I'm told it's better than having to use sonar just to get down the street." There weren't many ponies out so early in the evening, but a few stopped and stared. Not all of them at Twilight. She turned and saw Pinkie fluorescing—pink of course—as they passed by one patch of deep purple fungi.

Pinkie didn't seem to notice herself. “So is an archbishop like a normal bishop, except it moves in bits of circles instead of diagonal lines?”

Twilight summoned her notepad for a moment. “I’m going to have to look into that one. Moondancer keeps saying that normal chess is boring. But no, the archbishop is the religious leader of Canterburied, and one its most respected community members. The mayor's just a high-ranking civic functionary. Silent Night is who we'll need to talk to to make sure this is all above board.” After a moment, one that Twilight knew Pinkie was letting hang, she added, "So to speak."

Another turn brought them to a solid rock wall covered countless bas-reliefs of the moon in all its phases, constellatory creatures like the Ursa Major and Minor, and of course Luna, wings spread over the whole lot. The pony-sized door at the base almost seemed like an afterthought.

They stopped for a moment to consider the grand edifice. "Welcome to the Cathedral of the Heavenly Panoply. Luna never could get them to stop worshiping her." Twilight shrugged her wings. "Personally, I don't think she tried very hard."

Pinkie just tilted her head back to take it all in. "Oooooh."

“Now Pinkie, remember, this is a place of great solemnity and sanctity.”

That got her the most humorless look she'd seen out of Pinkie in years. “Twilight, I know important rocks when I see them.”

"Right." Twilight offered a sheepish grin. "Sorry."

The inside of the cathedral was almost as impressive, a tremendous vault that was only possible when every pony who had helped carve it had been able to fly. More glowing crystals twinkled overhead in perfect replications of the constellations... for now.

"Everything okay?"

Twilight realized she was frowning. "Stellar drift does mean that those will eventually grow inaccurate."

A nudge at her side brought her attention to Pinkie's grinning face. “Now Twilight, remember, this is a place of great solemnity and sanctity.”

They shared a giggle. “I deserved that. It’s just—”

“You cannot tell from down there, but the ceiling has a honeycomb pattern. We can slide out the gems and move them as the heavens shift.” Archbishop Silent Night drifted down to the floor, his glittering blue and black vestments fluttering in the descent. It was hard to tell with chiroptera, but the gray streaks in his mane seemed to be as much due to age as anything else. The crinkling around his eyes as he smiled certainly supported that. “Your Highness. I wasn’t expecting you so soon. Nor with Miss Pie in tow.”

Pinkie puffed out her chest. “Mrs.”

He nodded at the correction. “My belated congratulations. To what do I owe the pleasure?”

“I’m sure I don’t need to tell you about the upcoming…" Twilight looked for a diplomatic term and realized she already had one. "Scheduling conflict.”

That got a nod. “A helpful euphemism for a most unfortunate confluence, yes. I take it we will not have the pleasure of your attendance at the,” and here the archbishop’s mouth kept moving, but Twilight couldn’t register whatever sounds he made.

She shook her head. “We’re actually planning on a shared celebration. One that can bring together all ponies of the city, above and below.”

"Stars’ Warming Festeve!" added Pinkie.

"We're still workshopping the name."

Archbishop Night's eyebrows rose. “This is rather short notice, Your Highness. Both festivals begin in less than a week.”

Twilight shrugged her wings. “Pinkie and I only devised the plan earlier today. And while we have our best ponies on it, I’m sure you will agree that having an actual chiropteron perspective during the planning process is preferable to the alternative.”

Cheese frowned at the blackboard Lemon had wheeled in from the Twilight-established strategic blackboard reserve to help them draw up plans. “I’m just saying, I’ve never seen snow during a Festival of Stars.”

The breathing exercise Cadence had taught her was the only thing keeping her from strangling the stallion. “Because you were in a tropical jungle!”

“Hey, they’ve had snow, just not during the festival." Cheese tapped his chin in thought. "Though that might have just been because Dr. Caballeron was trying to pawn off the Hieratic Halo of Habakkuk the week before.”

Lemon blinked and started walking around the throne room, head on a swivel.

“What are you looking for?” said Cheese.

“An Erlenmare flask. If I’m going to get this much of a headache, it may as well have a bit of nostalgia to it.”

Archbishop Night nodded. “I see. Your intentions, at least, are noble, Your Highness. What have the Canterlot gentry said about this?”

“Ohhhh, right." Pinkie tilted her head in thought. "We should probably ask them at some point, huh?”

That got more surprise from the archbishop than anythng they'd told him thus far. “You haven’t already?”

Pinkie shrugged. “I just figured that anypony who’s anypony and who doesn’t come to the princess’s party isn’t anypony anymore.”

"This shift imposes on the chiroptera far more," said Twilight. "It’s only right that I came to you first.”

Archbishop Night hummed to himself. “You’re serious about this.”

Drawing herself up into the most princessly posture she could muster, Twilight nodded. “It’s a matter of friendship, Your Excellency. I couldn’t be more serious about it if an overdue book were involved.”

Pinkie leaned in close and stage whispered, “That’s really serious for her.”

“I gathered as much," said Archbishop Night, flashing a fanged smile that was gone the next moment, replaced with just as much severity as Twilight. "Still, you are a stranger to many of my people. They will not be easily convinced of this sincerity. Not without a greater showing of it.”

The two held each other's gazes for a few moments. Twilight sagged first. “Your Excellency, could we keep any grand quests or tests of worth to a minimum? As you noted, the festivities begin in less than a week.”

“We could bang one out in that time,” said Pinkie.

“Sure, one. But we wouldn’t have much time for the actual party planning.”

That got a shrug. “Depends on the quest.”

“Ladies?" Both turned to the archbishop. To Twilight's relief, he was smiling again. "You will no doubt be glad to know my request is a simple one: Do not let the link between the two sides of Canterlot end with the holiday. If you can, do not let them begin there either.”

An idea occurred to Twilight. She only needed one glance to know it had come to Pinkie as well. She sighed. “I may have a solution there. If an unorthodox and possibly offensive one.”

“I should think I’m the one who needs to concern himself with orthodoxy, Your Highness," said Archbishop Night. "Let’s hear it.”

Twilight looked around the castle ballroom. It was a fascinating blend of the expected and the novel from any perspective. Wreaths and the Hearth's Warming tree mingled with star crystals and displays of fruit that were meant to be eaten as much as admired. Consulting with performers above and below had yielded a song list that had those musicians alternating before familiar classics and freeform riffs on the songs of the other side. And, of course, the guests formed a menagerie of all of ponykind, and many other creatures besides.

It was the nonpony inclusions who had really helped grease the wheels. Especially Dragonlord Ember, who had taken one look at the chiroptera, grinned, and shouted "Finally! Ponies with real wings!" After that, it was impossible for any creature to worry that they'd be the first one to say something awkward, and the mingling had begun in earnest.

It almost seemed a shame to break the momentum. Still, Twilight floated up a cider glass and tapped with it a spoon until she had everycreature's full attention. "Fillies, gentlecolts, and friends of all species, thank you for coming to the first Winter Lights Ball."

"I still like my name better," she heard Pinkie mutter, along with Cheese's softer reassurances.

"We come here to celebrate the season with friends, for no matter what we celebrate, we come together in these long nights and appreciate the light we can bring to them by coming together. This is a time to bridge divides and forget past grievances, and I am thrilled that our friends in Canterburied agreed to join us at such an early hour.

"I won't keep you long. I simply want to wish everycreature a wonderful time tonight." As the guests provided polite applause, Twilight permitted herself a hint of sincere amusement in her regal facade. "Also, dinner-slash-breakfast is served."

That got a much more enthusiastic response as a tide of bodies made its way to the buffet, where Lemon Hearts led the kitchen staff in unveiling a number of dishes Pinkie and Cheese had devised in a furious multi-day experimental frenzy. Still, a few lingered here and there.

"A fine speech, Your Highness," said Jet Set, who Twilight suspected got more nutrition from brown-nosing than actual food.

"Thank you. I hope you and Upper Crust are enjoying the festivities?"

"Oh yes, quite. She's four months along and has had a positively furious appetite for durian. When she heard the bat—" The stallion stopped himself short once he spotted Twilight's warning glare. "Excuse, when she heard the chiroptera would be in attendance, she was even more insistent that we come."

"Congratulations," Twilight said sincerely. A foal was always a joyous occasion, regardless of what she might think of the parents.

"Thank you. We're thinking of Trust Fund if it's a colt and Bourgeoisie if it's a filly."

"... Ah."

Something like a thought passed through Jet Set's mind, which might have explained the look of discomfort. "Though... A question, if I may, Your Highness?"

That got a nod. "Of course."

He bit his lip, took a step back, and asked, "Is it contagious?"

Twilight considered her leathery wings, the fangs poking against her lower lip, the apple superimposed over her cutie mark.

And, too high for Jet Set to hear, she laughed.

Author's Note

Yes, Pinkie and Cheese have married at this point, but Li'l Cheese is still a ways out. We're only two or three years into Twilight's reign.

Let us never forget Flaskhead Hearts.

And yes, Twilight turned herself into a vampire fruitbat-pony in one comic. It was weird. Likewise the final arc of the comics, where Canterlot nearly got shaken off the mountain by an earthbending half-pony, half-lion.

In any case, Zaid asked for Pinkie and Twilight, and I conveniently had the vague concept of an idea involving them and bat pony diplomacy. Once I realized that the first night of Hanukkah falls on the 25th this year, I had the rest of the inspiration I needed. I've meant to write a blog about Canterburied for years. Actually making a story out of the idea instead of many past world building thoughtdumps is the surest sign of a Jinglemas miracle.

Finally, see this Dueling Keyboards short for a deleted scene.