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 the dead became thin on Samhain, allowing spirits to walk the Earth. People wore costumes and carved turnips to ward off spirits. As Christianity spread, the holiday blended with All Saints' and All Souls' Day, becoming Halloween. Immigrants brought Halloween traditions to America in the 19th century, where trick-or-treating and carving pumpkins instead of turnips became common practices still celebrated today.