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."
"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.
And off she trots, following Silver Streak's directions to her sister's place to issue a reminder about clearing the sky.
Just as Streak promised, there is a tower on the far side of town, some distance down the road from the main cluster of buildings and set a little ways back from the street. As Clarity approaches, mysterious sounds emerge: clanging, clinking, and a crackling whoosh almost like a strong wind.
"Just a miiiinute!" calls a voice that is similar to Streak's, but used very differently. In four syllables, this pony has varied her tone more than Streak did in her and Clarity's entire conversation.
"Hi!" she says cheerfully. "What's up?"
"Hi, you must be Brightblaze. I'm Clarity Bell, and the Princess sent me here to oversee the preparations for the Summer Sun Celebration -"
"Oh gee, that's today?" she says. "That's totally today, isn't it."
"Well, it's technically very early in the morning tomorrow, but the sky should be cleared, and you're on my list as responsible for that."
And she disappears back into the tower. A moment later, the clanging resumes.
Clarity isn't sure whether that was intended to be the end of the conversation or not, so she stays put for another minute in case Brightblaze is going to come back of her own accord.