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."
Clarity looks down at her front to see if there are crumbs or bits of dressing in her fur, but sees nothing. "Uh? ...Next I'm planning to look in on the decorations, to give whoever's clearing the sky extra time to get it done before I'm breathing down their mane." She peers up at the sky; it's not exactly crowded with clouds, but it doesn't look cleared.
"That would be my sister Brightblaze. When you're done inspecting the decorations, you'll find her in the tower on the far side of town, immersed in some project and having thereby forgotten what time it is and possibly what day. But it will not take her long to clear the sky, so as long as somepony reminds her with a few minutes to spare, all will be well."
Clarity writes down these directions. "If you're that sure she's forgotten perhaps I should just visit her next anyway."
"The town hall is directly between here and home, along the main street; you won't lose anything by stopping there first," she explains.
"All right. Well, don't let me keep you from rehearsing any more," says Clarity, and she turns to go.
"Goodbye," says the pegasus, and she picks up her violin again and turns her attention back to the choir.
Off Clarity trots towards the town hall. She is well ahead of schedule and should have plenty of time to research the warning.
The town hall looks pretty decorated! Appropriate sun symbol banners are hung from hither and yon, and streamers and crepe paper are strewn and elaborately knotted around other things, and the unicorn who seems to be handling this is in the process of arranging flowers to tuck into convenient loops of ribbon left when the bows were tied. "Oh! Hello!" she exclaims when Clarity approaches. "I'm Guiding Star." She slots a bouquet of camellias into a bow and cinches it up, then nods at her work and lets the golden magic fade. "You must be Clarity! Joy said you were going to come check things over. What do you think?"
"It looks pretty close to done, very tasteful, very neat... What else are you planning to add in the time you have left?"
"Well, all the bows are getting flowers. Do you think we've gone overboard on the colors? The florist thought we were going overboard on the colors. The idea is that the main color in every bunch of flowers matches its ribbon and the others complement it, and as you can see the ribbons are arranged in rainbow order."
"I probably would've been skeptical if I'd gotten a pitch in advance but it seems to be working okay in practice. Is the interior similar?"
"Oh yeah! Come on in." Guiding Star leads Clarity into the hall, which is indeed decorated similarly to the outside - "But we went with slightly darker colors because it's supposed to be nice and dim in here for the sunrise to be all the more dramatic, you know?"
"Oh yes. My special talent is seeing the future! Something could still go wrong, my spell's not perfect, but I know we can get everything done well in advance because I checked."
"Well, it takes a little while to cast, and then I get a picture of a likely future around wherever I'm going to be or sometimes somewhere else if I try really hard! It's basically 'the future if everything goes according to plan', I think, but I've never had a serious magic-type pony go over it with me. It can get thrown way off if somebody makes a choice that wasn't just about decided on when I cast the spell. Joy's trying to tone down his wackiness for the day so I can use the spell to see how the decorations will turn out before we invest all the labor in putting them up, but usually he throws me way off, and other ponies can too."
"Nope! If you're from Canterlot you must have left hours ago, the spell's had plenty of time to catch up. Wanna see me cast it?"
"Yep, everything looks great, except it looks like I was at risk of confusing the roses and the daisies, whoops! I'll just put those up right now in their correct places." Guiding trots out and then back in, a bouquet of each in tow, and tucks them into their ribbons one and then the other. "Oh, if you wanted to know, the choir goes up there on that balcony, and the catering tables are over there."
The named party sticks his head and one arm in an open window, high above them. "I'm up here, beribboning the roof! Hi!" He waves.