Evolving from the ancient Celtic holiday of Samhain, modern Halloween has become less about literal ghosts and ghouls and more about costumes and candy. The Celts used the day to mark the end of the harvest season and the beginning of winter, and also believed that this transition between the seasons was a bridge to the world of the dead. Over the millennia the holiday transitioned from a somber pagan ritual to a day of merriment, costumes, parades and sweet treats for children and adults.