Where did Halloween come from?

Halloween, or Hallowe'en, is a holiday celebrated on the night of October 31, most notably by children, who, in a tradition commonly known as trick-or-treating, dress in costumes and go door-to-door to collect sweets, fruit, and other gifts. Other traditional activities include costume parties, viewing horror films, visiting "haunted houses", 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, after which Irish and Scottish immigrants carried versions of the tradition to North America in the nineteenth century. Several other western countries have embraced the holiday as a part of American pop culture in the late twentieth century. Halloween is now celebrated in many parts of the western world, most commonly in the United States, Canada, Puerto Rico, Ireland, the United Kingdom and sometimes in Australia and New Zealand. In recent years, the holiday has also been celebrated in some 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[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 we now consider All Saints' (or Hallows') Day 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.