The Christian holiday of All Hallows' Eve, celebrated on the same date, was originally celebrated on May 13, and was moved to its present date by Gregory III in the eighth century.
The site also links to a wide selection of Halloween-related web pages, covering All Hallows' Eve subject matter from ghost stories to costumes to "mystic games" like a virtual Ouija board and a tarot card reading.
Both All Hallows' Eve -- known today as Halloween -- and All Saints' Day, which follows on November 1, were designated as days to pray for dead souls that had not passed out of purgatory, in the belief that the prayers of the living will help.