Halloween originated as a pagan festival known as Samhain. It was celebrated by the ancient Celts in Ireland, Scotland, and parts of England. Samhain marked the end of the harvest season and the beginning of winter, and it was believed to be a time when the boundary between the living and the dead was blurred. The Celts would light bonfires and wear costumes to ward off roaming spirits during this festival. Over time, as Christianity spread, elements of Samhain merged with Christian traditions and eventually evolved into the modern-day Halloween celebration.