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."
"Okay, you can all come if you want, but - at your own risk. I feel bad enough that I didn't successfully get Celestia's attention with my warning; I'm sure you all know, living right here, more about the forest than I do, but still." Here's the library; Clarity opens it up and collects her saddlebags.
"Believe me, I'm aware of the risks." She cocks an ear at the sky and adds, "Oh good, Streak must have heard us."
Guiding gallops up presently too. "Why are we all going to the Everfree Forest with Joy?" she demands.
"We're going to find some mysterious objects Clarity read about that she thinks might help defeat the scary night pony," says Cordy.
"Okay, but Joy'd better behave himself or I'm not going to be that much help tracking them down - I wonder where he is."
"Well, he wasn't on the shortlist for invitations, so if we just head for the forest now he'll probably find us on the way, won't he?" reasons Clarity, and she starts off.
"Hey, Blaze, hey, everypony!" he says. "Where are you all headed?"
"We're going to the Everfree Forest to find artifacts that may or may not still be there, may or may not prove usable, and may or may not help, and apparently you're joining us."
"Guiding, you want to take a moment to make sure we're all intact in a while?"
Guiding stops, casts her spell, and presently reports: "I see us all in the forest with all our limbs and everything. Joy, behave, okay?"