All Hallows Eve

On Samhain, or All Hallows Even, spirits were able to pass from the underworld into the world of the living.

Since both good and bad spirits would come through, the good ones were allowed to go home with their living family members, and the bad ones were warded off by people dressed up in costumes

Source: http://blogs.babble.com/famecrawler/2010/10/31/all-hallows-eve-halloween-wasnt-always-for-kids/