that special day of the year when girls (or usually just one girl and her friend) as an inside joke, dress as slutty as possible to make fun of all of those other bitches who do it daily, thinking they look so hot. *also to see how many guys get "excited" if you know what i mean ;]*
synonyms: Halloween, All Hallow's Eve. An American holiday celebrated on October 31 for which women, particularly of the college-going age, dress like sluts by wearing themed costumes.
See "slutty Halloween costume formula."
My girlfriend bought a great French maid costume for National Dress Like a Slut Day; mini-skirt, fuck-me pumps, and fishnet stockings.