Usually she'd put comparatively little stock in millennium-old legends, but this is a primary source, and the authors made some correct predictions on other matters, and the translator is good, and she double-checked to make sure the page numbers matched up so no one has been pranking her with inserted extra leaves. And the prediction is fairly dire.
And it's rather short notice.
Clarity tucks the book into a saddlebag and trots up to the main section of the palace. She can usually get an audience fairly quickly; Celestia has taken a particular interest in her since her admission to the School for Gifted Unicorns. This day is no exception. In fact, the guards usher her in even quicker than she'd usually expect.
"Princess, I have something urgent to tell you," Clarity says, mid-bow.
"Oh? What is it, Clarity?" inquires Celestia.
"I've found a prediction stating that on the longest day of the thousandth year -"
"Clarity," sighs Celestia.
Clarity grits her teeth; she's never told Celestia not to interrupt her. You can't tell the princess that. "Yes, Princess?"
"I have the utmost respect for your diligence at your studies, but you've got to take your nose out of those dusty old books now and again," says Celestia with a warm half-chuckle. "There's more to a young pony's life than studying. You're in the prime of your life, you're coming up on the end of what you can learn from school -"
Clarity stands up on all fours, no longer bowing, jaw slightly open.
"And," continues the Princess, "you need to make some friends. I worry about you all alone, you know. Weren't you invited to that party - who was it, Moondancer?"
"Yes, I was invited, but I barely know her," says Clarity.
"Perhaps the ponies around Canterlot just aren't clicking with you. You know what," says Celestia, as though seized with sudden inspiration. "I'm going to give you a job to do."
"...a job, Princess? But what about the longest day of the thousandth -"
"Clarity," says Celestia again, and Clarity shuts her mouth. "Don't you worry about it. I'm sending you to supervise the preparations for the Summer Sun Celebration. It's being held in Ponyville this year. I'll arrange a carriage for you. Pack your things, you leave in two hours."
"I..." Clarity can't really gainsay her. "Yes, Princess."
Celestia smiles.
Clarity bows again, and leaves, and packs.
-
She's in the air, drawn with her luggage in her carriage by someone in good condition for a long-distance flight - they're still working on that train line.
She steps onto the main street of Ponyville, laden with all her possessions in her saddlebags, and levitates her to-do list in front of her nose.
"Okay," she sighs to herself, peering at the first item. Clarity is pretty sure Celestia has read every book in the palace library at least once; probably she investigated the danger well in advance and dismissed it. (But Clarity will be staying in a library while she's in Ponyville, so she'll have a chance to double-check and find corroborations once she checks out the Summer Sun Celebration preparations.)
"First," Clarity mutters, "banquet preparations. Cheery Cherry Orchard."
"I guess the library would have to stay unstaffed for quite a while to still have the position available in a few years, though." Clarity locates the disorganized shelves and scans them in case there's something else Elements- or Mare-in-Moon related but finds nothing.
"Well, you clearly know how to perform at least one librarian-designated task already, I would have spent hours looking for that book on my own."
Clarity is going to do some notebooking now until it is time to congregate at the Summer Sun Celebration.
When the time of the celebration approaches, a pink earth pony with a green mane comes to collect her. Blueberry puts her book away and trots up to her with a happy cry of, "Hi, Clover!", then follows her to the town hall.
It is crowded, the food is tasty, everypony in town is mingling, the choir stands ready, and the sky is still completely clear to allow proper viewing of the sunrise. The decorations are finished and the curtain is down to allow proper dramatic reveal. A mare who must be the Mayor is standing by it to present Celestia at the correct moment.
"Fillies and gentlecolts, as mayor of Ponyville, it is my great pleasure to announce the beginning of the Summer Sun Celebration!" cries the mayor. "In just a few moments, our town will witness the magic of the sunrise, and celebrate this, the longest day of the year! And now, it is my great honor to introduce to you the ruler of our land, the very pony who gives us the sun and the moon each and every day, the good, the wise, the bringer of harmony to all of Equestria... Princess Celestia!"
That's the cue for the songbirds, and for the drawing of the curtain.
"Remain calm, everypony!" exclaims the mayor. "I'm sure there's a reasonable explanation!"
Clarity suspects there is an unreasonable explanation. She is debating whether to teleport up there and double-check for herself when -
"Oh, my beloved subjects. It's been so long since I've seen your precious, little sun-loving faces."
"Your subjects?" cries an indignant pegasus. "What the hay did you do with our Princess?"
Her sister yanks her out of the air by her tail and sits on her to prevent further outbursts.
The newcomer chuckles. "Why, am I not royal enough for you? Don't you know who I am? Does my crown no longer count now that I have been imprisoned for a thousand years? Did you not recall the legend? Did you not see the signs?"
"I know who you are," Clarity says loudly before the alicorn starts trying to forcibly remind anypony of her identity. "You're Nightmare Moon."
This time Clarity doesn't answer fast enough to suit her.
"Remember this day, little ponies," laughs Nightmare Moon, "for it was your last. From this moment forth, the night will last forever!"
She cackles, and thunder echoes across the cloudless night sky.
Some policeponies charge the alicorn, but she just laughs and takes off out the window into the dark.
Clarity forces herself not to break into a canter that will guarantee her a timewasting trip over her own hooves as she makes for the exit. She doesn't know any part of Ponyville well enough to teleport there without looking straight at it in better light than that of the stars and the suddenly blank-faced moon, so on hoof it'll have to be. She doesn't know for sure if the contents of her saddlebags are going to be useful in the Everfree Forest - actually, she'd put her bits on 'no' - but while this is an emergency, it is not an emergency on a scale of minutes and so she's going to go ahead and take the precaution.
"What do you know about this that I don't?" inquires Cordy.
"Nightmare Moon equals Mare in the Moon equals banished sister of Celestia, could theoretically be defeated with five mysterious and one extra mysterious objects that can maybe be found in an old castle in the Everfree Forest," Clarity recites, not breaking stride.
"And you mean to go get them? Sounds reasonable. I'll help plan the expedition," says Cordy. "We should bring Guiding Star and Silver Streak for informational purposes, and Brightblaze would be good to have along in case of trouble, plus there's no way she'll stay behind if her sister is going."