What did the Celts believe happend on the last day of october each year?

The Celts believed that on the last day of October, specifically during the festival of Samhain, the boundary between the living and the dead became blurred. They thought that on this night, the spirits of the dead could return to the earth, leading to a time of heightened supernatural activity. To honor these spirits and protect themselves, the Celts would light bonfires and wear costumes, which eventually influenced modern Halloween traditions.