Halloween originated from the ancient Celtic festival of Samhain, celebrated on October 31st. The Celts would wear costumes and light bonfires to ward off ghosts and spirits that they believed returned on this night. As Halloween spread, it incorporated elements from the Christian holiday All Saints' Day. Today, Halloween is celebrated in many countries with costumes, parties, trick-or-treating, and activities like apple bobbing. Popular costumes include witches, ghosts, vampires and monsters.