Hates Halloween. Passionately. He hates costumes and dress-up parties. And for the love of God don't mention murdered blondes on Halloween. Just... point Sam at something that needs hunting or killing and stay out of his way.

That said, he has worn any number of costumes to go undercover. My personal favorite is Priest!Sam, but I don't think even Dean could talk him into dressing up for Halloween. Unless they needed to sneak into a cathedral.

Also, blonde ladies, please don't anyone dress up as a sexy nurse. He'll lose his shit completely. (By which I mean, please, go right ahead.)