Halloween (hălˌəwēnˈ, hälˌ–) [key], Oct. 31, the eve of All Saints' Day, observed with traditional games and customs. The word comes from medieval England's All Hallows' eve [Old Eng. hallow = saint]. However, many of these customs predate Christianity, going back to Celtic practices associated with Nov. 1, which was Samhainsäˈwĭn, the beginning of winter and the Celtic new year. Witches and other evil spirits were believed to roam the earth on this evening, playing tricks on human beings to mark the season of diminishing sunlight. Bonfires were lit, offerings were made of dainty foods and sweets, and people would disguise themselves as one of the roaming spirits, to avoid demonic persecution. Survivals of these early practices can be found in countries of Celtic influence today, such as the United States where children go from door to door in costumes demanding "trick or treat."
See N. Rogers, Halloween: From Pagan Ritual to Party Night (2002), D. J. Skal, Death Makes a Holiday: A Cultural History of Halloween (2002).