Halloween or Hallowe'en[7][8] (less commonly known as Allhalloween,[9] All Hallows' Eve,[10] or All Saints' Eve)[11] is a celebration observed in many countries on 31 October, the eve of the Western Christian feast of All Hallows' Day. It is at the beginning of the observance of Allhallowtide,[12] the time in the liturgical year dedicated to remembering the dead, including saints (hallows), martyrs, and all the faithful departed.[3][13][14][15] In popular culture, the day has become a celebration of horror, being associated with the macabre and supernatural.[16]

One theory holds that many Halloween traditions were influenced by Celtic harvest festivals, particularly the Gaelic festival Samhain, which are believed to have pagan roots.[17][18][19][20] Some go further and suggest that Samhain may have been Christianized as All Hallow's Day, along with its eve, by the early Church.[21] Other academics believe Halloween began solely as a Christian holiday, being the vigil of All Hallow's Day.[22][23][24][25] Celebrated in Ireland and Scotland for centuries, Irish and Scottish immigrants took many Halloween customs to North America in the 19th century,[26][27] and then through American influence various Halloween customs spread to other countries by the late 20th and early 21st century.[16][28]