“The day originated in Ireland and Scotland with the Celtic and Druid pagans. They believed the most active night of the spirit world was October 31. On that day every year Chron, the sun god, died. Samhain, their god of the dead, could bring back to life Chron, but only if people would sacrifice innocent blood.
Most Halloween symbols reflect these pagan origins. The jack-o-lantern, for instance, originally was a large turnip or skull with a candle in it. Some believe the fearsome face represented Samhain, who would drive off less powerful spirits. Others say the terror-stricken face represented a man named Jack whose soul wandered about between heaven and hell, unable to reside in either.
Similarly, trick-or-treating originated when people dressed in costumes in order to fool the evil spirits about their true identity, and thus cheat death and avoid the wrath of Samhain. People went “asouling" to neighbors, demanding favors and food and threatening to cast a spell if their demands were not met."