Once upon a time, in the land of Equestria, on a night where the air was quiet and the land was sleeping, a grand carnival was held in the streets of Canterlot, the royal city of the two princess alicorns. Much to their shock, everypony whom entered the city gates, be they young or old, was turned into a mere child. Everypony played and danced and sang and feasted deep into the night. When the clock struck twelve, however, the Carnival ceased and everypony would be restored to their original age, be they young or old.
For more than one thousands years, this event, dubbed the Endless Carnival, has mysteriously appeared in the city of Canterlot, on the same day every one hundred years. Princess Celestia was confused by this, no doubt, but whoever, or whatever, brought this glorious celebration every century, they would bring great happiness to everypony. Though most merely dismissed it as a dream, they still felt great happiness after the night was through.
"My Dearest Princess Twilight Sparkle. I would like you and all of your friends to join me in Canterlot this weekend..."