Dressing up in costumes and going "guising" (trick or treating) was prevalent in Scotland at Halloween by the late 19th century. Costuming became popular for
Halloween parties in the US in the early 20th century, as often for adults as for children. The first mass-produced Halloween costumes appeared
in stores in the 1930s when trick-or-treating was becoming popular in the United States.
Halloween costume parties generally fall on, or around, 31 October, often falling on the Friday or Saturday prior to Halloween.