[Leo has a feeling the party was thrown by Lucifer when he got multiple invitations with the dress code underlined. He didn't want to go, but when he realizes that the journal is not going to stop annoying him, he decides it's the best chance to annoy Lucifer too.
So he picks the oldest clothes he can find and goes to the party straight to where Lucifer is to make sure he sees him. Sooner or latter Miracle Country will change him into a proper outfit but damn it if he's not going to try to get back at his roommate a little.]