Halloween originated from the ancient Celtic festival of Samhain, held on October 31st to celebrate the end of the harvest season and honor the dead. The Celts believed that on Samhain, the boundary between the living and the dead became blurred, allowing spirits to return to Earth. People would wear costumes and masks to imitate and placate the spirits. Over time, Halloween evolved from a Celtic religious festival to a secular celebration involving costumes, games, and carving jack-o'-lanterns.