There's a disguise party at Ponyville, and everypony is there, dressed as whatever they want. The main goal? The best costume will get a prize.

While Twilight is busy at the library, Spike and the others go for spending some time at the party.

Wait, who said Twilight was actually at the library?

This short story takes place during the first seasons, when Twilight Sparkle is still a Unicorn.