Halloween originated from the ancient Celtic festival of Samhain, celebrated on October 31st, which marked the end of the harvest season and the beginning of winter. The Celts believed this was a time when spirits could cross over into the human world, so they would light bonfires and wear costumes to ward off spirits. As Christianity spread across Europe, elements of Samhain blended with Catholic holidays honoring saints and souls of the dead, eventually becoming the Halloween celebration known today involving traditions like jack-o-lanterns, trick-or-treating, and costume parties.