It's Wednesday right now, and Halloween is Saturday. If you don't have a costume and you're exceptionally lazy, go as an undercover cop like I did last Halloween and wear the stuff you normally wear. Two people will be laugh, the host/hostess will be pissed, and someone will definitely call you an asshole. However, if you've got a suit lying around the house, and you can be Megadeth mascot Vic Rattlehead for Halloween!
The mask is currently going for about $47 on Amazon, so you might want to hurry up with ordering it. This way everyone at the Halloween party can ask if you're some formal skeleton or something, and then the one guy who has a vague awareness of what metal is can ask if you're that guy from Metallica.
It'll be fun, I promise.