On May 13, A.D. 609, Pope Boniface IV dedicated the Pantheon in Rome in honor of all Christian martyrs, and the Catholic feast of All Martyrs Day was established in the Western church. Pope Gregory III later expanded the festival to include all saints as well as all martyrs and moved the observance from May 13 to November 1.
By the 9th century, the influence of Christianity had spread into Celtic lands, where it gradually blended with and supplanted older Celtic rites. In A.D. 1000, the church made November 2 All Souls’ Day, a day to honor the dead. It’s widely believed today that the church was attempting to replace the Celtic festival of the dead with a related, church-sanctioned holiday.
All Souls’ Day was celebrated similarly to Samhain, with big bonfires, parades, and dressing up in costumes as saints and, angels. The All Saints’ Day celebration was also called All-Hallows or All-Hallowmas, and the night before it, the traditional night of Samhain in the Celtic religion, began to be called All-Hallows Eve and, eventually, Halloween.
