That's some pretty fancy evening wear, with Twilight Sparkle's being especially playful. It looks very comfortable and natural on her, and she probably doesn't even give it a second thought since she's just happy to be goofing around with her friend. Rarity is a bit more stylish and reserved, but certainly not unhappy to play along and enjoy the night too. As a matter of fact, I'm guessing she's hoping to stay up late since she's drinking some joe... er,Nice work on the really cute corner ponies too.