Halloween costumes are outfits worn on or aroun October 31, the day of Halloween. Halloween is a modern-day holiday originating in the Pagan Celtic holiday of Samhain (in Christian times, the eve of All Saints Day). Although popular histories of Halloween claim that the practice goes back to ancient celebrations of Samhain, in fact there is little primary documentation of masking or costuming on Halloween before the twentieth century. Costuming became popular for Halloween parties in America in the early 1900s, 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.