Halloween is said to stem from the Celtic festival of Samhain, which was where people would construct bonfires and dress up in costumes to keep evil spirits away. Pope Gregory III (8th century) made November 1 All Saints Day to memorialize saints and martyrs. The night before All Saints Day was then known as All Hollow's Eve, then Halloween. Over the years, it became a more community-involved event with the addition of trick or treating.