One of the rituals adopted from the Celts was pumpkin carving, which held religious significance. “The jack-o'-lantern custom consists of placing fire—which imitates the good magic of the sun—inside a hollowed out vegetable, representing the harvest,” Suppe says. It was done “in hopes that the good magic will help to preserve the harvested food through the dark half of the year, until the next growing season could replenish the community's food stocks.”
Later, in Ireland and Scotland, people developed the custom of using similarly-carved vegetable lanterns to scare off the mythical character of Stingy Jack, who wandered the Earth because the devil wouldn’t let him into hell.
Similarly, “the practice of trick-or-treating originates in the Celtic custom of giving token bits of the harvest to spirits wandering outside of houses on the evening of Samhain, to placate them and prevent them from doing destructive things to the harvest or to homes,” Suppe says. Once Christianity became established in the Celtic regions, young, unmarried men would parade around on Halloween, going to houses and calling for gifts to the spirits.
“This was a time when the hard work of the harvest was done, so they could indulge in some pranks to let off steam,” Suppe says. In Scotland, the groups of young men were called “guisers” (pronounced “GEE-zers”) because they wore disguises—the beginning of the custom of wearing Halloween costumes.
Centuries later, Halloween customs were brought to the United States by immigrants from Ireland, Scotland and other ancient homelands of the Celts. As an 1894 article in Christian Work describes the holiday: Halloween is a night "when witches, evil spirits, and all mischief brewing sprites went forth on dark and mysterious midnight revels.”