Halloween, or Hallowe'en, is a holiday celebrated on the night of October 31. Traditional activities include trick-or-treating, costume parties, Halloween festivals, bonfires, visiting "haunted houses", viewing horror films, and participating in traditional autumn activities such as hayrides (which may have "haunted" themes).

Halloween originated under the name of Samhain as a Pagan festival among the Celts of Ireland and Great Britain. Irish and Scottish immigrants carried versions of the tradition to North America in the nineteenth century. Other western countries embraced the holiday in the late twentieth century.

Halloween is now celebrated in parts of the western world, most commonly in Ireland, the United States, Canada, Puerto Rico, the United Kingdom and sometimes in Australia and New Zealand. In recent years, the holiday has also been celebrated in various other parts of Western Europe.

The term Halloween (and its older rendering Hallowe'en) is shortened from All-hallow-even, as it is the evening of/before "All Hallows' Day", also known as "All Saints' Day". It was a day of religious festivities in various northern European Pagan traditions, until Popes Gregory III and Gregory IV moved the old Christian feast of All Saints' Day from May 13 to November. In the ninth century, the Church measured the day as starting at sunset, in accordance with the Florentine calendar. Although All Saints' (or Hallows') Day is now considered to occur one day after Halloween, the two holidays were, at that time, celebrated on the same day. Liturgically, the Church traditionally celebrated that day as the Vigil of All Saints, and, until 1970, a day of fasting as well. Like other vigils, it was celebrated on the previous day if it fell on a Sunday, although secular celebrations of the holiday remained on the 31st. The Vigil was suppressed in 1955, but was later restored in the post-Vatican II calendar.