Halloween or Hallowe'en (a contraction of All Hallows' Evening), also known as Allhalloween, All Hallows' Eve, or All Saints' Eve, is a celebration observed in a number of countries on 31 October, the eve of the Western Christian feast of All Hallows' Day and the Reformation Day. It begins the three-day observance of Allhallowtide, the time in the liturgical year dedicated to remembering the dead, including saints (hallows), martyrs, and all the faithful departed.

It is widely believed that many Halloween traditions originated from Celtic harvest festivals that may have pagan roots, particularly the Gaelic festival Samhain, and that this festival was Christianized as Halloween. Some academics, however, support the view that Halloween began independently as a solely Christian holiday.