When did Halloween begin the merge with Catholic festivals?

Halloween began to merge with Catholic festivals around the 7th century when Pope Boniface IV established All Saints' Day on November 1st, originally intended to honor saints and martyrs. This date coincided with the Celtic festival of Samhain, where people celebrated the end of the harvest and believed that the boundary between the living and the dead was blurred. Over time, the customs and traditions of both Samhain and the Christian observance evolved, leading to the modern celebration of Halloween.