People wear costumes on Halloween because of a custom that dates back more than 2,000 years to Samhain, the Celtic festival marking summer’s end, when people disguised themselves as beasts or monsters to blend in with the spirits they believed roamed the earth that night.
The Celts believed the barrier between the living and the dead grew thin on the eve of Samhain, which fell around November 1, so they also left out food to pacify fairies, demons and other visitors. In the seventh or eighth century, the church’s All Saints’ Day, or All-Hallows, took over that date, and the night before became All Hallows’ Eve, or Halloween. By the 16th century, children in Scotland and Ireland went “guising,” making the rounds disguised as ghouls and receiving treats meant to ward off malign spirits. Irish and Scottish immigrants brought the custom to the United States in the 1840s. The term “trick-or-treating” appeared in the 1920s, and costumed trick-or-treating took off after World War II, when sugar rationing ended.
Source: Smithsonian Magazine, “The Celtic Origins of Trick-or-Treating”
Image: Girls in Halloween costumes, about 1914–1918, Richard from USA, CC BY 2.0, via Wikimedia Commons.