Halloween (All Hallows Eve) is the evening of October 31. In its strictly religious aspect this occasion is known as the vigil of Hallowmas or All Saints' Day, November 1, observed by the Roman Catholic and Anglican churches. In the fourth decade of the 8th century, Pope Gregory III assigned this date for celebrating the feast when he consecrated a chapel in St. Peter's basilica to all the saints. Gregory IV extended the feast to the entire church in 834. In Latin countries the evening of October 31 is observed only as a religious occasion, but in Great Britain, Ireland, and the United States, ancient Halloween folk customs persist alongside the ecclesiastical observance.

Students of folklore believe that the popular customs of Halloween show traces of the Roman harvest festival of Pomona and of Druidism. These influences are inferred from the use of nuts and apples as traditional Halloween foods and from the figures of witches, black cats, and skeletons commonly associated with the occasion. In pre-Christian Ireland and Scotland, the Celtic year ended on October 31, the eve of Samhain, and was celebrated with both religious and harvest rites. For the Druids, Samhain was both the "end of summer" and a festival of the dead. The spirits of the departed were believed to visit their kinsmen in search of warmth and good cheer as winter approached. It was also an occasion when fairies, witches, and goblins terrified the populace. The agents of the supernatural were alleged to steal infants, destroy crops, and kill farm animals. Bonfires were lighted on hilltops on the eve of Samhain. The fires may have been lighted to guide the spirits of the dead to the homes of their kinsmen or to kill and ward off witches.

During the middle ages when the common folk believed that witchcraft was devoted to the worship of Satan, this cult included periodic meetings, known as witches' Sabbaths, which were allegedly given over to feasting and revelry. One of the most important Sabbaths as held on Halloween. Witches were alleged to fly to these meetings on broomsticks, accompanied by black cats who were their constant companions. Stories of these Sabbaths are the source of much folklore about Halloween.

Pranks and mischief were common on Halloween. Wandering groups of celebrants blocked doors of houses with carts, carried away gates and plows, tapped on windows, threw vegetables at doors, and covered chimneys with turf so that smoke could not escape. In some places boys and girls dressed in clothing of the opposite sex and, wearing masks, visited neighbors to play tricks. These activities generally resembled the harmful and mischievous behavior attributed to witches, fairies, and goblins. The contemporary "trick or treat" custom resembles an ancient Irish practice associated with All Hallows Eve. Groups of peasants went from house to house demanding food and other gifts in preparation for the evening's festivities. Prosperity was assured for liberal donors and threats were made against stingy ones. These contributions were often demanded in the name of Muck Olla, an early Druid deity, or of St. Columb Cille, who worked in Ireland during the 6th century. In England some of the folk attributes of Halloween were assimilated by Guy Fawkes day celebrated on November 5. Consequently Halloween lost some of its importance there.

Immigrants from Great Britain and Ireland brought secular Halloween customs to the U.S., but the festival did not become popular in this country until the latter part of the 19th century. This may have been because it had long been popular with the Irish, who migrated here in large numbers after 1840.