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Nothing could be further from the truth. The origins of Halloween are, in fact, very Christian and rather American. Halloween falls on October 31 because of a pope, and its observances are the result of medieval Catholic piety.
It’s true that the ancient Celts of Ireland and Britain celebrated a minor festival on October 31--as they did on the last day of most other months of the year. However, Halloween falls on the last day of October because the Feast of All Saints, or "All Hallows," falls on November 1. The feast in honor of all the saints in heaven used to be celebrated on May 13, but Pope Gregory III (d. 741) moved it to November 1, the dedication day of All Saints Chapel in St. Peter’s at Rome. Later, in the 840s, Pope Gregory IV commanded that All Saints be observed everywhere. And so the holy day spread to Ireland.
The day before was the feast’s evening vigil, "All Hallows Even," or "Hallowe’en." In those days Halloween didn’t have any special significance for Christians or for long-dead Celtic pagans.
In 998, St. Odilo, the abbot of the powerful monastery of Cluny in southern France, added a celebration on November 2. This was a day of prayer for the souls of all the faithful departed. This feast, called All Souls Day, spread from France to the rest of Europe.