Halloween
Tradition of Dressing in Costume
halloween
So people dressed up as scary creatures / ghouls / etc. with the idea that if the real ones saw them they would be fooled and leave them be. The idea was that ghosts / ghouls / scary nastiest. Don't go around scaring each other - that's no fun - so if you looked like one of them you would be relatively safe from their antics.
http://wiki.answers.com/Q/What_is_the_tradition_of_dressing_up_at_Halloween_hope_to_do