Bam bam bam! Somecreature knocked on Twilight's front door.
As usual, Spike was the creature who opened it. He found a charcoal-colored pony wearing a pink bowtie.
"Huh," Spike said. "Whaddaya want, Octavia?"
The mare smiled. "I'm here as a Thursdayist. May I speak with Princess Luna, please? I understand she is visiting Ponyville this weekend."
Spike rubbed his face with his claws. "I don't know. Wait here while I find out."
Inside the Castle of Friendship, Spike trudged along hallways, and up and down stairs. After a few minutes, he found Luna in the kitchen, wearing a fuzzy bathrobe and eating fruit.
"Good afternoon, Spike."
"Hi Luna. Somepony's at the front door wanting to talk to you."
"Did they say what it's about?"
"She said she was a Thursdayist."
Luna snorted. "Let Us find out what that is."
***
The front door reopened. "Greetings, my little pony."
"Princess Luna!" The visitor fell to her knees. "What an honor it is to finally meet you. I've been dreaming of this moment for most of my life."
Luna chuckled. "Oh, We think We would know if that were true. Thou seemeth to be...twenty-five years old at least. Within thy lifetime, hardly anypony had ever heard of Princess Luna before four or five years ago."
"Oh, but I beg to differ."
Luna raised an eyebrow. "Dost thou?"
"I am a Thursdayist. We believe every Thursday, the world is destroyed and created anew. So my entire life began on Thursday."
Luna snorted again. She looked the visitor in the eye. She slowly circled the charcoal-colored pony, inspecting her from every angle. Finally, the princess said, "Thou art serious?"
"But of course, your highness. I would not take the holy name of Thursdayism in vain."
Luna walked back around to sit on the ground in front of the visitor. "Any point in Our mentioning that We remember the last week like We were there?"
"Of course you do," the bowtied pony said. "When we're created on Thursday, we're created with memories of the earlier weeks. That's a great blessing. We learn from the experiences of other ponies who came before us. And other ponies, who come after us, will learn from our mistakes without being guilty of our sins."
"We do not see what difference it makes."
"The difference is, YOU never did those things, Luna."
"So?"
"You don't have to feel guilty about them. You just have to learn from the experience of the pony who DID do those things, who wasn't you."
"Hmm." Luna wiggled and stretched. "Kind of freeing, in a way. We get the benefit of experience, without having even DONE those things."
"Yes. Your sins are washed away, Princess Luna. You never did those things. You just have to keep from doing those things now, in this lifetime."
Luna snorted. "Until next Thursday, anyway."
"Well, sure. Next Thursday is a totally new life."
"And if We felt guilty about events before last Thursday?"
"Not your fault."
"And if We...felt so bad about the things We THOUGHT We had done before last Thursday, that We wanted to create a creature out of dreamstuff to punish Us in Our sleep?"
"Sounds like a bad idea to me, Princess Luna. If you feel bad about something your previous counterpart did before last Thursday, just be careful not to do those things again. And if there's anything you can do to fix those things, fixing them might be a good idea. But don't punish yourself for things that somepony else did."
"Is that all there is to it?"
"Just accept that everything before last Thursday is in the past, and isn't about you anymore. And if your past counterparts made mistakes, try to do better than they did."
"Hmm." Princess Luna smiled. "Wouldst thou like to come in for breakfast?"
"Breakfast? Isn't this dinnertime?"
"I work nights."
"Oh. Of course." The charcoal-colored pony smiled back. "Please forgive me for making assumptions."
"Trust Us. So many times before, ponies have made so many terrible assumptions about Us--I mean, about Our past counterparts. We doubt thou shalt do anything worse than Our past counterparts have experienced time and time again."
***
In the kitchen, Luna and the door-to-door evangelist conversed enthusiastically over a fruit bowl.
"So thou seest," Luna said, "if only We had gotten the word long enough ago, We would have understood there was no POINT in trying to punish ponies for having gone so many years not loving Luna and the night. Because for almost all of that time, We were not the pony they were failing to appreciate. And it was different ponies who had done most of it anyway, not the ponies who were around when We turned into Nightmare Moon."
"You get it!" the charcoal pony said. "I wish everypony would understand Thursdayism so quickly!"
Luna frowned. "So many ponies suffered so very much, to make Us the pony full of guilt and shame who was so happy to hear thy message. If this princess accepts the Word easily, it is at a great cost."
The evangelist said, "But now you've learned your lesson, right? You won't take away the sun and plunge the world into darkness forever?"
Luna chuckled. "Of course We have learned Our lesson. It is fundamentally impossible for Us to make that mistake, because the longest time We could keep the sun from coming up would be a week. Any longer than that, and it would no longer be Us doing it."
Octavia made a funny expression. "I suppose that IS logical. But I don't think THAT'S really the main point."
Luna laughed, "Ah ha ha HA HA HA HAHAHAHA! Enjoy life while thou canst. For next Thursday, your entire life shall be over. YOLOW!"
Octavia said, "Yolow? What does that mean?"
"You Only Live One Week!"
Octavia seemed somewhat alarmed. "I really think before you try to fully apply the teachings of Thursdayism, you should at least finish reading the pamphlets to find out what those teachings ARE."
"Nonsense! Give Us these pamphlets for future reference, though." Luna's powerful magical aura grabbed some documents away from her visitor.
"Princess Luna? One of those was my electric bill."
"We shall take care of it for you. That is the LEAST We can do, for the pony who showed Us the Light. We mean, the Darkness. Same difference."
"Princess Luna, I'm not supposed to accept personal favors like that while I'm proselytizing. Our leader says it could be a conflict of interest."
"Do not worry about it," Luna said. "For now, thy princess must go speak with some other ponies."
With a loud poof and a flash of light, Luna was gone.
"Hmm," Octavia muttered. "Well, I guess as long as those OTHER ponies bother to read the pamphlets all the way to the end, it should be ok."
***
In Twilight's downstairs lab, Luna appeared with a flash of light.
"Hello, Luna. What brings you here so early in the afternoon?"
Luna held out a pamphlet. "Hast thou ever considered that the thou BEFORE last Thursday, and the thou AFTER next Thursday, are in fact completely different ponies, and not the same pony as thou at all?"
Twilight set her test tube down in a rack. "Yes, I learned that the hard way. I got all upset over some news from the me of next week that turned out to be nothing, and I put everypony in Ponyville through a lot of trouble and stress, and I made a complete fool of myself and tried to burglarize the Royal library in Canterlot to get a time travel spell."
"We see. Thou must be very happy to know that it was not thou who did any of those things. But We shall leave thee a pamphlet anyway." Luna vanished.
Twilight blinked. "Wasn't ME? Wait, WHAT were you trying to tell me?" Twilight shook her head. "And you aren't even here to answer my questions about whatever it was you just said. You are SO different from your sister. More than anypony else, you remind me of MYSELF from a week in the future."
Twilight noticed the new pamphlets on her lab bench. "But you left me SOMETHING TO READ. I'm so excited!"
***
Celestia sat in the palace gardens, sipping tea.
With a flash and a boom, a dark blue alicorn appeared in front of her.
"Luna? I thought you were visiting Twilight in Ponyville this weekend. And I can't quite put my hoof on it, but you're looking a bit different from usual."
"We WERE in Ponyville. But We have more important matters to attend to. For now We have become THURSDAY MOON, and the relative lack of guilt and shame shall last until NEXT THURSDAY! Or possibly forever, depending on what Our future counterparts do."
Celestia took another sip of tea, and put her cup down on the table in front of her. "I see, dear sister. But I fear you have been misinformed."
"Misinformed? How dost thou fail to accept the truth of Thursdayism?"
Celestia smiled. "While you were away on the moon, many things have changed here in Equestria. It is a shame that you still haven't quite caught up on things."
Something about Celestia seemed a bit different. She wasn't in her Daybreaker form...but she seemed just slightly closer to it than usual. "For I," Celestia shouted in the Royal Canterlot voice, "am TUESDAY NOON! And THOU? Thou art a HERETIC!"
A beam of golden light shot from Celestia's horn, exploding the ground beneath her possibly well-meaning alicorn sister. "And THAT was but a warning shot. Yield, heretic, and accept the TRUTH! Or I shall smite ye so hard, ye shall be blown into NEXT TUESDAY!"
"Nay!" Luna cried in her own Royal Canterlot voice. "Into next THURSDAY!" She shot a warning beam of her own, destroying Celestia's teacup.
"Next TUESDAY!" Celestia's beam barely skimmed Luna's chestpiece, smudging it with a bit of soot.
"Next THURSDAY!" Luna struck beneath her sister with just enough force to blow the pale alicorn up and out of her shoes.
The two sisters took to the air, swooping and weaving and dodging over Canterlot Palace as they shot scary-looking beams at each other.
Down on the ground, a pink alicorn walked up to the Palace's main entry. Guards saluted her and asked what they might do for the Princess of Love.
The pink alicorn blushed. "I WAS going to show Princesses Celestia and Luna this fascinating pamphlet somepony gave me recently, about the spiritual significance of Wednesdays." Cadance looked up at the aerial battle overhead. "But it seems the two sisters are busy right now. Maybe I should try back another time."
Author's Note
I did NOT invent Last Thursdayism. But have I perhaps PERFECTED it? 
(Cover Image: I clowned around with emoji in a Discord channel.)
