Celestia is tired of ruling Equestria. So she decides to take a day off... using any means possible.
In this case it involves pretending to be sick and tricking her sister into ruling in her stead for the day.
Well, her clever plan succeeds, and Celestia is left to herself all day long. As she is enjoying her time off, she discovers a stack of letters in her bedroom; letters that she hasn't thought about in a very long time.
Letters from Rainbow Dash.
Celestia begins to read these letters from so many years ago. They are full of Rainbow's passions, her hopes, her dreams, and her love - her love for one of her best friends: Applejack. They follow Rainbow's relationship with Applejack, blossoming from friendship into something more, resulting in cuddles and kisses and eventually in Rainbow Dash learning some of the most important lessons that anypony can ever learn.
The last letter from Rainbow had been an invitation to a wedding. And it had been sent a year ago.
So why had Rainbow Dash stopped writing?