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...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"[1], also known as "All Saints' Day". It was a day of religious festivities in various northern European Pagan traditions[citation needed], until Popes Gregory III and Gregory IV moved the old Christian feast of All Saints' Day from May 13 to November 1. 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.

In Ireland, the name of the holiday was All Hallows' Eve (often shortened to Hallow Eve), and though seldom used today, the name is still well-accepted, albeit somewhat esoteric. In Irish, the festival is known as Oíche Shamhna (Night of Samhain), or simply Samhain; in Scottish Gaelic it is Samhainn or Samhain; in Welsh, Calan Gaeaf to the Welsh; "Allantide" to the Cornish and "Hop-tu-Naa" to the Manx. Halloween is also called Pooky Night in parts of Ireland, presumably named after the púca, a mischievous spirit.

Many European cultural traditions hold that Halloween is one of the liminal times of the year when spirits can make contact with the physical world, and when magic is most potent (according to, for example, Catalan mythology about witches and Irish tales of the Sídhe).