Halloween originated from the ancient Celtic harvest festival of Samhain. The Celts believed the boundary between the living world and the spirit world became thin on Samhain, allowing spirits to cross over. Christians later incorporated aspects of Samhain into All Hallows' Eve, celebrated on October 31st. Over time, All Hallows' Eve evolved into Halloween with traditions like wearing costumes and trick-or-treating. Today, Halloween maintains folk traditions mixed with modern secular celebrations and superstitions.