Samhain became the Halloween we are familiar with when Christian missionaries attempted to change the religious practices of the Celtic people. In the early centuries of the first millennium A.D., before missionaries such as St. Patrick and St. Columcille converted them to Christianity, the Celts practiced an elaborate religion through their priestly caste, the Druids, who were priests, poets, scientists and scholars all at once. As religious leaders, ritual specialists, and bearers of learning, the Druids were not unlike the very missionaries and monks who were to Christianize their people and brand them evil devil worshippers.
As a result of their efforts to wipe out "pagan" holidays, such as Samhain, the Christians succeeded in effecting major transformations in it. In 601 A.D. Pope Gregory the First issued a now famous edict to his missionaries concerning the native beliefs and customs of the peoples he hoped to convert. Rather than try to obliterate native peoples' customs and beliefs, the pope instructed his missionaries to use them: if a group of people worshipped a tree, rather than cut it down, he advised them to consecrate it to Christ and allow its continued worship.
In terms of spreading Christianity, this was a brilliant concept and it became a basic approach used in Catholic missionary work. Church holy days were purposely set to coincide with native holy days. Christmas, for instance, was assigned the arbitrary date of December 25th because it corresponded with the mid-winter celebration of many peoples. Likewise, St. John's Day was set on the summer solstice.
