Celestia patiently maintained her smile while she sat upon her throne, as Parallel Parking, whose special talent lay with the construction of parking lots, enthusiastically attempted to convince her of the importance and need of parking space throughout Equestria.
While she tried to be respectful, by listening to what was being said, she couldn't help wondering how she would say what needed to be said without crushing her little pony's spirit. Unfortunately, there just wasn't much demand for parking space in Equestria, in large part because most ponies did not own their own means of transportation, relying more on public transportation and private services.
Before it came time for her to speak, however, everyone's attention was drawn to the flash of light that suddenly appeared in the center of the hall. It heralded the unexpected entrance of her sister, Luna, whose stony gaze was immediately focused upon her. She began to feel uneasy in reaction, her mind racing to divine what could have upset her, because there was no doubt about her mood. What had her sister encountered while she had been familiarizing herself with the advancements of their society and the settlements that had arisen over the past thousand years?
"Leave us," Luna tersely told the room, her eyes never straying from her sister.
Getting the hint, both Parallel Parking and the guards made quick their exit without question or complaint, leaving the two princesses alone. Celestia didn't speak straight away, nor did her sister elect to do so herself in the seconds that passed in silence, while she tried to think of what could have inspired her sister to direct her ire toward her. There were a number of things that certainly could have been responsible, but they had either been resolved at some point, hidden away, or probably wouldn't have been discovered by her sister during her excursion. She began to wonder if she might have forgotten something, considering how long a thousand years was.
Deciding to not beat around the bush, she calmly observed, "You are upset with me."
Without preamble, Luna began to channel magic through her horn, and a second later an old-fashioned, wooden outhouse appeared beside her. Upon seeing it, and the moon-shaped hole on the upper portion of the door in particular, Celestia stiffened but otherwise managed to maintain her composure. If one were to look closely, however, they would see sweat already beading on her brow.
Pointing at the outhouse in a swift, jabbing motion, Luna flintily asks, "Why is it that this... ignoble utility bears my symbol?"
Celestia continued to stare at the utility in question, wondering where it could have come from. It had taken her a while, but she had been certain that her efforts to phase out the crescent moon had been successful. She could only guess that there had been a need to build an outhouse in one of the frontier towns, and the responsibility had fallen to an older pony who either didn't have the skill to cut out the town's emblem or had done it as an independent worker, thus having no obligation to use a business' logo.
Returning her attention to Luna, she could tell that she wouldn't tolerate any attempt to act ignorant or apologize and simply put the matter behind them. There was a fair chance that she had learned something about the connection between the outhouse and the moon anyway, so saying anything irrelevant or false might add more fuel to the fire. Hopefully, if she could get her sister to understand what she had done in the past, she wouldn't be too upset with her.
With that thought in mind, she deflated a bit and released a sigh of resignation. "Before I explain, I want you to know that I only had the best of intentions in mind at the time." Seeing that her sister was about to say something, and not something nice based on the look on her face, she quickly added, "Please hear me out before you pass judgement."
Luna paused and regarded Celestia with a thoughtful frown, but eventually acquiesced and gestured for her to continue with a nod of her head.
Celestia closed her eyes and spoke in a deliberate manner, voicing over the images that played out behind her eyelids. "After I banished you to the moon, I was... emotionally distraught. For a time I locked myself away in my quarters, lamenting what had happened, and wondering how to move forward." She paused and opened her eyes, so she could stare into her sister's. "Eventually, after thinking back on what you had said right before your transformation into Nightmare Moon, it occurred to me that I could help our little ponies appreciate your nights."
Looking at her with a dubious expression on her face, Luna asked, "Is that how Nightmare Night came about?"
"No," Celestia replied, with a rueful shake of her head. "That came later, but was nonetheless a consequence of what I did as well."
Despite being somewhat worried about what she would hear, Luna's curiosity and need for an answer compelled her to prompt her sister onward. "Go on."
Celestia complied after she took a few seconds to recompose her thoughts. "I decided that the night should be celebrated, and made it a national holiday. I was... very enthusiastic about it, to say the very least. Although I did it for your sake, in hindsight I feel that it had been as much for mine as well."
Not liking the direction that her sister seemed to be going with her explanation, Luna hesitantly asked, "What did you do?"
Lowering her head in shame and embarrassment, Celestia reluctantly replied, "I chose the harvest moon to be the day of the festivities, when food and drink would be most plentiful. At first, our little ponies were sympathetic and wouldn't deny me anything." She sighed and cast her gaze aside. "Unfortunately, I was rather insistent that they eat and drink to their heart's content. I joined them, of course, and, well..."
It took Luna a moment to connect the outhouse to the excessive eating and drinking. When she did, she regarded her sister with an incredulous look and said, "Please tell me you're joking." When her sister shook her head, she could hardly believe it as she asked, "How many times did this happen?"
"Enough times to inspire Nightmare Night later on," Celestia admitted. Seeing the confusion on her sister's face, she explained, "Eventually ponies stopped attending the festival with increasing regularity, and after it reached the point where I made a habit of inviting former attendees to the festival personally, they began to disguise themselves — in various ways — in order to avoid me." With a look of remorse, she added, "Because it was easier for them to do that than to turn me down, or stand up and tell me what I was doing wrong."
After a moment of contemplation, Luna relaxed her posture and expression. "I suppose I can overlook what you caused, considering the circumstances." After a brief pause, which was barely enough time for her sister to perk up, she pointedly asked, "However, you did try to stop this from becoming a trend, correct? This isn't normal?"
With evident relief, Celestia sat up straight and confidently replied, "Don't worry: I am making sure that it will be lost to the annals of history." With a frown, she added, "There just hasn't been enough time for it to completely fade from memory. Frankly, I'm surprised that this even happened."
Satisfied enough with that answer, Luna zapped the crescent moon on the outhouse into Dodge Junction's emblem, then magicked the outhouse back to where it belonged. With that done, she returned her attention to her sister and casually said, "Well, as long as that is your only fault, I shan't hold it against you. I imagine that your past shenanigans affected you more than I, anyway."
Celestia simply nodded her head in agreement, because she didn't trust herself enough to speak. While her scandalous behavior had certainly created problems, they hadn't been as much of an issue as her sister probably assumed, which was largely attributed to her charming persona and the forgiving, understanding nature of their subjects. Also, it hadn't hurt that there had been someone to blame for her actions. That was why Nightmare Moon had been cast as the villain of Nightmare Night, rather than herself.
Before Luna could finish her teleportation spell and leave, to continue her tour of Equestria, she recalled something that made her cancel it. "Oh," she voiced, her expression brightening, "I learned of this phrase yesterday, 'over the moon,' and I was curious about its origin. Would you happen to know?"
Caught by surprise, due to the subject matter, Celestia coughed behind a fetlock and tried to buy herself enough time to figure out what to say. "Ah... 'Over the moon.' Well..."
Although they had been separated for a thousand years, and were still in the process of becoming reacquainted, Luna was able to discern what her sister's reaction meant. As such, she narrowed her eyes in suspicion and inquired, "I take it that this phrase has a history that I would find objectionable?"
Celestia weighed her options before resigning herself to telling the truth, which was partly inspired by the dangerous glint that she caught in her sister's eye once her hesitation had become apparent. So, with a single, reluctant nod of her head, she confessed, "That phrase has the same origin as the crescent moon on the outhouse, I'm afraid." She wasn't surprised when her sister prompted her to elaborate. "As you can no doubt imagine, many ponies did not feel well after the festivities. Eventually, when they 'got over it,' they began to say that they were 'over the moon.' In time, it changed to mean getting over anything horrible, before simply becoming an expression of great happiness."
With a huff, Luna irritably remarked, "There wouldn't happen to be anything else that tarnishes my beloved moon, would there?" When her sister avoided eye contact and didn't answer right away, it was with a good amount of disbelief that she eventually uttered, "There can't be more... Can there?"
Deciding that it would probably be best to come clean, instead of having her sister find out that she was hiding things of concern to her later, Celestia steeled herself, returned her gaze to the fore and regretfully replied, "Quite a few, I must confess."
Luna furrowed her brow and drew her lips into a thin line, speaking through her teeth as she queried, "Such as?"
Celestia took a moment to suppress the nervousness that her sister inspired in her before replying. "Well... Because of my behavior shortly after your banishment, our ponies began to associate lunacy with the moon, and called those suspected of being affected by it — like they thought myself — moonstruck." Wanting to soften the blow, it was with an optimistic tone that she added, "Oh, but it can also refer to somepony daydreaming about another romantically!"
Seeing that her sister was even more upset about being associated with insanity, and was seemingly unmoved by the secondary usage of "moonstruck," she pressed onward, hoping to reel back her anger with sympathy. "Also, if somepony is 'crying for the moon,' they are making an unrealistic request. It had been inspired by the many nights I had spent gazing up at the moon, where — in my grief — I'd said many things that had been overheard from my balcony. That's also where the phrase 'moon something away' comes from, because of how long I had withdrawn myself from everything."
While her sister's facial features softened, which was promising, there was still one more expression to reveal, that she had saved for last because she was almost certain that it would be the worst of the lot: not only due to how it might affect her sister, but because she found it the most embarrassing, personally. As such, her reluctance to share it caused her to duck her head and look askance while she repeatedly pressed the undersides of her forehooves together.
"There's one last thing," she temporized. "And I want you to bear in mind that I was rather inebriated when I inspired this. In fact, it happened during one of the few festivals that I have little recollection of."
Upon checking on her sister, and seeing that she was frowning impatiently, she closed her eyes and shrank into herself a bit before blurting out, "I bared my posterior in a shameless and disrespectful manner! And because some of those present likened what they saw to the moon, henceforth was such an action called 'mooning!'"
She chanced opening one eye when her sister didn't answer in a timely fashion, and saw that she was standing unnaturally still, her face frozen in a blank expression. Then, suddenly, her sister's wings shot out as far as they could be spread, and her hair erupted into twin pillars of impending twilight as she shouted, "What!?"
Celestia flinched at her reaction, but quickly got to her hooves so she could begin her "battle" to get back into her sister's good graces. "Luna, allow me—"
"Silence!" Luna bellowed, reaching decibels that exceeded the "royal Canterlot voice," which shook the hall and caused many cracks to blossom in the stained glass windows. With her eyes aglow with power, she stomped with enough force for cracks to spread beyond the carpet that she stood upon and declared, "This is an outrage! As if I didn't have enough to be ashamed about already, because of my own actions. I won't be burdened by your own mistakes, as well!"
Before Celestia could make another attempt at discourse, she had to stop and look on in horror as a familiar transformation began to play out in front of her. When it ended, what she saw gave her reason to pause for a second: because, while the mare that now stood before her was recognizable as Nightmare Moon, her attire was not the same. Instead of wearing armor, she wore a dark blue, star-spangled robe that covered all but her hooves below the neck. Around said neck was a chain, and from it hung a tome. And to top it all off, she wore a pair of thick-rimmed glasses.
She wasn't about to allow herself to be distracted by the change, so she stood her ground and glared defiantly while doing her best to ignore her sorrow and quell the desire to blame herself. "Nightmare Moon..."
Looking affronted, "Nightmare Moon" thrust a foreleg at Celestia and declared, "My name is Nomenclature Moon! And when I'm finished dragging your precious 'sun' through the mud, just the mere utterance of the word shall cause those who hear it to be stricken with nausea!"
Nomenclature Moon proceeded to laugh maniacally as she shifted into a floating, amorphous blob of disembodied hair and made her exit via the space beneath the hall's double doors.
Dumbfounded, Celestia stood motionless for a while, trying to accept what had happened while wondering about the future. Eventually, she broke the heavy silence of the room by collapsing upon her haunches, releasing a tired sigh and muttering, "Maybe there is something to the notion that the moon can have adverse effects on the mind."
Author's Note
I was researching the exact nature of the rearing pony silhouette that can be seen in various places, on Ponyville's flag in particular, and wondered why it was on an outhouse in another town. Things just kind of snowballed from there. 
