Halloween originated from the ancient Celtic festival of Samhain, celebrating the end of the harvest season and the beginning of winter. Celts believed the boundary between the living and dead became blurred on Samhain, when spirits visited Earth. They wore costumes and built bonfires to ward off spirits. The traditions were influenced over time by Roman festivals and Christianity's All Saints' Day, becoming the secular Halloween celebration today involving costumes, trick-or-treating, and community events.