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Why do people wear costumes on All Hallow's Eve?
The superstition was that the visiting ghosts could disguise themselves in human form, such as a beggar, and knock on your door during Samhain asking for money or food. If you turned them away empty-handed, you risked receiving the wrath of the spirit and being cursed or haunted.
Another Celtic myth was that dressing up as a ghoul would fool the evil spirits into thinking that you were one of them so that they would not try to take your soul. In the U.S., trick-or-treating became a customary Halloween tradition around the late 1950s, after it was brought over by Irish immigrants in the early 1900s.