“Start at the beginning,” Celestia said. She waved a hoof at the tea set in front of her. “Perhaps a sip of tea will calm your nerves.”
Twilight snatched the cup from the table, poured a hoof's height into it despite its shivering in mid-air, and shotgunned it all in one swoop. Still a bit senseless, Twilight babbled, “It's just, I trust my memory for everything I do, and it can't be failing me now...”
“I'm sure your memory is just as good as always,” Celestia said.
“That's just it! I can't remember last week!” Twilight replied.
That caught Celestia's attention. “...Please. Tell me when you first noticed this problem.”
“It was… Tuesday, I think. I'm already starting to have trouble remembering, because I still feel like that was yesterday, but today's Tuesday as well, and--”
“...Right. So, it was Tuesday, and I was spending time with my friends. We were at the Star Greens Cafe; I don't know if that's important, but I should mention it in case it is.”
“It's probably not,” Celestia said. “Go on.”
“So I was sitting there, and we were chatting about nothing particularly important. Pinkie was telling us about her new shipment of balloons, I think, when I suddenly started feeling… different.”
“Different?” Celestia asked. “Different how?”
“Like… like I was falling asleep, or like I was suddenly really dizzy. Or… both. It's not something I've really felt before. Anyway, my vision started to blur, but none of the others seemed to notice anything wrong. And then… and then it was just this morning!”
“This morning, as in today?”
“Right.” Twilight shook her head and poured herself another cup of tea. “It wasn't teleporting; it wasn't time travel… I thought it might be a sudden-onset case of Dissociative Identity Disorder, but I talked with my friends, and they said I was completely the same as usual!”
“But you don't remember anything from the past week, though?” Celestia added.
Twilight sighed. “That's the thing. I'm starting to remember things. Bits and pieces, really. Cooking dinner for Spike one day, or meeting Fluttershy to do her animal census. I remember doing them. I remember being there. But I don't remember… doing them. Do you know what I mean?”
“I believe so,” Celestia said solemnly. “It seems like a different you altogether who did those things.”
“Yes!” Twilight sighed with relief. “So you do know what's going on, then?”
“I think so.” Celestia stood up from her little tea party chair and turned to look out at the horizon, where her sun was midway towards sunset. “My sister and I call it 'Alicorn Time.'”
“Your sister and… you mean you've experienced it, too?” Twilight asked.
“I have, and the descriptions Luna's given me of her own condition seem to suggest the same diagnosis.” Celestia tapped a hoof against the ground and looked to a corner, deep in thought. “If Cadance has had it, she hasn't mentioned it to me yet.”
“What is it?” Twilight asked. “Is it some kind of amnesia, or a sickness?”
Celestia shook her head. “No. Luna and I… we always called it a defense mechanism. A safeguard to protect the mortal pony brain from the pains of immortal life.”
“I don't understand,” Twilight said.
“Twilight.” Celestia spoke softly, resigned. “Eternity is a very, very long time. If our brains attempted to remember every single thing that happened to us, we'd run out of space. Memory's much more complicated than that, of course, but that's sort of the gist. Our brains simply can't handle millions, billions of years of memories all stacked up on top of each other. So for us alicorns, our brains… cheat a little.
“It's like… imagine a story where all of the pages are the same. Would you read every single page, when you already know what the next twenty, fifty, one hundred pages are going to say? No. You'd start to skim. You'd jump from page to page, looking for the pages that are actually different. That's what our brains are doing: skimming all the time that is the same, day-in and day-out, and jumping to the moments that are different.”
Celestia sighed. “Your case is just the beginning. Your alicorn brain decided to jump a week, but your regular pony brain panicked, creating a different day to land on. It'll probably do that a few more times before it eventually settles in.”
Twilight was starting to hyperventilate again. “So you mean… I'm going to start forgetting all of my memories with my friends? But they were valuable to me! They mean everything to me!”
“I know, and I'm sorry,” Celestia replied. “You'll still get the particularly important days: the weddings, the births, the funerals. But most of the regular days, the everyday chatter and fun… no, most of that you will simply pass over, like flying over empty plains.”
Twilight had never looked at her life like that before. Book-sortings came by months, and birthdays by years; but in 10 years, the marriages; in 15, the births; in 60, the funerals… and then another set if she made new friends. “Won't my friends notice if I'm acting strange? That I'm not 'there' for years at a time?”
“You'll act exactly as you always have,” Celestia reminded her. “Haven't you ever been in a routine?”
“...Have you, Princess?” Twilight asked. “What is it like? For you?”
Celestia frowned. Slowly, she said, “I've ruled for one thousand years, and I remember almost none of it.” Twilight raised her eyebrows, and Celestia continued, “I remember the days just after Luna was banished. I was grief-stricken, and I could hardly get out of my bed. But duty called, and soon, I entered into the daily ritual of ruling an entire nation.” She paused and smiled, a pale smile that didn't reach her eyes. “I remember the first few weddings. I'd choose some noble who'd caught my eye, date for a year or two, then have an elaborate ceremony. The more elaborate, the more likely I'd remember. But even that became a routine.” She laughed and shook her head. “Marry 'em at 35, bury 'em at 75. Like clockwork.”
“How many times did you do that?” Twilight asked, aghast. “How many times did you marry somepony you didn't truly love?”
“But I did love them, don't you see?” Celestia said. “It was still me. I went through all the motions, and nopony was the wiser. I fell in love, courted, married, and mourned when they died.” She stamped on the ground again for effect. “Like clockwork.”
“So what moments do you remember?” Twilight asked.
“The wars,” Celestia replied. “Apparently, I never created a routine for that. The natural disasters—earthquakes, floods, volcanoes.” At Twilight's teary-eyed expression, Celestia quickly added, “But I have positive memories, too! The day Luna came back is like a perfect portrait in a museum to me. The day when I founded Ponyville, if you can believe it. But the days I remember best are the days I found my students.” She walked over and placed a gentle hoof on Twilight's shoulder. “Every student is different to me. I remember the day you turned your parents into potted plants, and the day Sunset Shimmer almost set fire to my castle. I remember all of them, uniquely and specially.”
“And all the days you taught me?” Twilight asked. Celestia hesitated, just for a moment. “Princess… did you ever go into Alicorn Time when you were teaching me?”
Celestia tipped her head to a side, her eyes betraying tears. “Now, Twilight, you know I can't control it. I appear exactly the same whether or not I'm in Alicorn Time.”
“Please, just tell me!” Twilight said.
“Whether or not I did, you know that I cherished every moment with you, that I loved you--”
“Please!” Twilight cried.
Celestia paused for a few seconds. Then, trying her best to smile, she said quietly, “We… had an awful lot of tea parties, didn't we, Twilight?”
Twilight stumbled back, and her teacup fell onto the grass, spilling its contents into an inky pool around the table. “All that time… all that time, and you were just… on autopilot?”
“Autopilot connotes a lack of emotion, Twilight,” Celestia said sternly. “I was just as emotional as anypony else. The love was real. I just don't remember.”
Twilight scoffed. “You loved me as a matter of routine,” she said.
“And what about your friends?” Celestia replied quietly. “Do you love them as a routine?”
“No, but I—I suppose, but—I won't let it happen again!” Twilight shouted.
“How exactly do you plan on doing that?” Celestia said.
“I'll make each day different! I'll make every picosecond count!” Twilight said.
“And how long can you sustain that?” Celestia replied. “Alicorn Time exists precisely because you can't do that forever. And just to throw another wrench into your plan,” she added, “how long will it take before 'doing something new and different' is your routine?”
“--!” Twilight stopped and fell onto the ground. She wanted to cry, but it seemed like all of her energy had drained away, and she now had nothing left to even spill tears. “So… that's it. There's no escape.”
“No.” Celestia relaxed her body and sat down once more at the table. “This isn't a curse, Twilight. It's our body's way of protecting us from keeping track of too much. You'd probably done it before you were an alicorn, too. Do you remember every single day of your classes?”
“Not really, no...”
“They all just blur together. But I'm sure, if you really tried, you might be able to remember living each day, one by one.” Celestia smoothed out her mane and poured out another cup of tea. “Remarkably inefficient, don't you think? Having to live through a whole day when you're just going to forget it anyway. It's a luxury you simply can't afford as an immortal.”
Twilight sat down again and, her magic wavering, took another cup and tried to drink. “So… are you in Alicorn Time right now?”
“If I am, I won't know until I land again, and you'll never know for certain.” She sipped from her tea, slowly, relishing it. “Probably not, though. This is, after all, a lifechanging moment for my most faithful student. But then again,” she added, “you've gone through a great many already.”
“...Oh.” Twilight sighed and drank from her tea again.
“Are you in Alicorn Time right now?” Celestia asked.
“...I don't think so. I think I'm conscious, right this moment.”
“Mmm.” Celestia savored that for the moment, then continued. “Will you remember this moment, then?”
Twilight looked around the garden. “I hope so. It's so nice and peaceful, and I've learned so much.”
“How long do you think you'll remember? A year? A decade? A millennium? Or is this one of the rare, precious few that will last forever?” Celestia's horn lit up, and the sun sank a bit faster than normal, beginning its descent into sunset. “After all, there are a great many beautiful sunsets.”
Subconsciously, Twilight began evaluating the evening. This sunset was nice, but was it as nice as the one last Monday, or the one the evening after they'd saved the Crystal Empire? Was it better than all the sunsets she'd already forgotten? Finally, all she could say was, “I don't know.”
“Aah. So perhaps by the end of the month, you'll have forgotten most of the specifics, and you'll only remember the key points.” Celestia shrugged. “Perhaps that will be more than I'll remember. I have so many more memories jostling for my attention. So many more sunsets.”
For perhaps the first time, Twilight saw Celestia not as wise or just or benevolent, but as simply old, a mare with millennia upon millenia of time poured out and lost already. She wasn't sure whether to pity Celestia or to respect her more for it. Instead, she just said, “You have a few you value the most, right?”
Celestia smiled, for real this time. “Mm-hmm. I'm glad for the memories I do have. For the time well spent.” But then her smile faded, and she looked wryly out at the horizon again. “But alas, even the best of times are spent, and we must a-bed. Goodnight, Twilight.”
She didn't remember saying goodnight back.