Halloween is a celebration observed in many countries on October 31st, the eve of the Western Christian feast of All Saints Day. It is the first day of Allhallowtide, a time in the liturgical year dedicated to remembering the dead, including saints, martyrs, and all the faithful departed. Halloween has its roots in the ancient Celtic festival of Samhain, which was held on November 1st in contemporary calendars. It was believed that on that day, the souls of the dead returned to their homes, so people dressed in costumes and lit bonfires to ward off spirits. Over time, Halloween became increasingly secular, and today it is considered a holiday for dress-up and fun, especially for children.

Popular Halloween activities include trick-or-treating, attending Halloween costume parties, carving pumpkins or turnips into jack-o-lanterns, lighting bonfires, apple bobbing, divination games, playing pranks, visiting haunted attractions, telling scary stories, and watching horror or Halloween-themed films. Trick-or-treating is a custom in which children dress up in costume and solicit treats from neighbors, and it became popular in the United States in the early 20th century as Irish and Scottish communities revived the Old World custom of “guising,” in which a person would dress in costume and tell a joke, recite a poem, or perform some other trick in exchange for a piece of fruit or other treat. Halloween parties often include games such as bobbing for apples, perhaps derived from the Roman celebration of Pomona.

The English word Halloween comes from "All Hallows Eve," being the evening before the Christian holy days of All Hallows Day (All Saints Day) on November 1st and All Souls Day on November 2nd. Halloween is thought to have influences from Christian beliefs and practices.