All Saints' Day by William Adolphe Bouguereau (1859)
From a Halloween History (although I don't think "syncretism" is the right word to use):
The Catholic celebration of All Saints' Day was officially inaugurated in 609 A.D., although the Christian influence on Halloween actually begins in 601 A.D., when Pope Gregory I instructed his missionaries that, rather than obliterate native peoples' customs and beliefs, they should try to use them; hence, Catholic holy days were set at the times of native holy days, celebrations and festivals. As Christian missionaries moved into Ireland, they practiced Gregory's doctrine of "syncretism" and replaced the Celts' Samhain with All Saints' Day (Pope Gregory III moved the observation to November 1 in the eight century). In 998 A.D. St. Odilo, Bishop of Cluny, instituted a day of prayer and special masses for the souls of the dead. This day became All Souls' Day, and was set for November 2nd. In many areas All Saints' Day and All Souls' Day celebrations merged, as in Mexico, where October 31 to November 2 became known simply as "Days of the Dead".