Halloween is popular in Ireland, where it is said to have originated. In the Irish language it is known as Oíche Shamhna or “Samhain Night”. Pre-Christian Celts had an autumn festival, Samhain, “End of Summer”, which was
a pastoral and agricultural “fire festival” or feast when the dead revisited the mortal world and large communal bonfires would hence be lit to ward off evil spirits. It is alleged that the Irish continued to practice their deep-rooted ancient pagan rites well after the arrival of Christianity in the mid sixth century.
Pope Gregory IV standardized the date of All Saints’ Day, or All Hallows’ Day, on November 1 in the name of the entire Western Church in 835. As it now began at sunset, the holiday coincided exactly with Samhain. Although there is no official documentation that Gregory had Samhain in mind when selecting this date, it seems consistent with the common practice of leaving pagan festivals and buildings intact while overlaying a Christian meaning. While Celts might have been content to move All Saints’ Day from the previous date of April 20th, they were unwilling to give up their pre-existing April festival of the dead and continued to celebrate Samhain.
Source: Wikipedia
