The celebration of Halloween has dual origins. The first is in a pre-Christian Celtic feast associated with the Celtic New Year. The second is in the Christian celebration of All Saints Day (Nov. 1st) and All Souls Day (Nov. 2). In the British Isles November 1st is called All Hallows, thus the evening before is All Hallows Eve.
The Celtic Feast
The ancient Celtic peoples who inhabited England, Wales, Scotland, Ireland and Brittany (NW France) celebrated their New Year's Day on what would be November 1st on our calendar. Prior to their conversion to Catholicism these peoples practiced a pagan religion controlled by a priest class known as Druids. The Druids are most famous for the stone monument of Stonehenge and other astronomical calendars that remain in their former domains.
The period prior to the New Year, as the year wound down, was a time to consider the mystery of human death. It was believed that on the last night of the year the lord of death, Samhain, allowed the souls of the dead to return to their homes. Souls that had died in sin, and in Celtic belief imprisoned in the bodies of animals, could be released through gifts to the lord of death, including human sacrifices. It was also thought that evil spirits, demons, ghosts, witches were also free to roam around this night and could be placated by a feast. They would also leave you alone if you dressed like them and thus appeared to be one of them. Families would also extinguish their hearth fires on this evening to be re-lit from a common New Year's bonfire built on the hilltops, which was meant to symbolize the driving away of darkness and evil with the coming of the new year. The jack-o-lantern as a means of scaring away evil and providing light may be a vestige of this custom. When the Romans conquered Gaul (France) and Britain (excluding Scotland and Ireland) in the century before and after Christ, the bloody elements of Druidic practice were banned.
The Christian Feasts of All Saints and All Souls
During the first three centuries of Christianity the Church frequently had to operate "underground" due to the persecutions of the Roman state against her. During these periods there were many martyrs who died for their faith in Jesus Christ. The most renowned of these were honored locally by the preservation of the relics (if available) and by the celebration of the anniversary of their death, as a feast in honor of their birth into eternal life. As time passed, neighboring dioceses would honor each others martyrs and even exchange relics for veneration, the way the first century Christians kept the clothes and handkerchiefs touched by St. Paul (Acts 19:12).
At the end of the third century and the beginning of the fourth the most vicious of all persecutions occurred, that of the emperor Diocletian (284-305). The martyrs became so many that in some places it was impossible to commemorate even the most significant of them. The need for a common feast of all martyrs was becoming evident. This common feast became a reality in some places, but on various dates, as early as the middle of the fourth century. As far as Roman practice goes it is known that on 13 May 609 or 610, Pope Boniface IV consecrated the ancient Roman Pantheon as a temple of the Blessed Virgin and All Martyrs. Beginning with Gregory III (731-741) the celebration of a feast of All Saints was commemorated at St. Peters on November 1. Gregory IV (827-844) extended this feast to the entire Church.
The feast of All Souls developed more gradually, first with a monastic celebration of their departed on October 1st. This seems to have occurred first in Germany in the 900s. The patronage of St. Odilio of Cluny extended this feast to other monasteries, first of his own Order, then to Benedictines and others, from where it spread to dioceses, including Rome. It was only in 1915 that the special privilege of three Masses was granted to all priests by Pope Benedict XV.
Halloween during Christian Times
The conversion of Celtic peoples to Christianity did not dampen their enthusiasm for the pre-Christian year-end custom of feasts, bonfires, and masks, essentially new year's eve costume parties. The proximity to the developing Christian feasts of All Saints and All Souls resulted in an attempt to move the celebration to the evening before All Souls, when children would go door to door receiving treats for a promise of prayer for the dead of the household. This attempt to associate the Celtic remembrance of the dead with the Christian memorial ultimately failed and the celebration remained a year-end custom (by the old Celtic calendar), though Halloween remains primarily a children's feast.
With the massive emigration of Irish in the last century the All Hallows Eve customs of costumes, jack-o-lanterns and trick or treating, were transported to North America. Scary costumes remain the historical norm for Halloween, though the advent of more sinister and violent times has encouraged many parents to take a gentler approach. Today many families, and even parishes, hold group celebrations, often with costumes of the saints, the poor souls or famous Catholics (such as the Pope, Mother Teresa or the like) and other elements which re-enforce the Christian side of Halloween's origins.