The celebration of All Saints Day dates back to the 7th century A.D., when the Roman temple known as the Pantheon was dedicated by Pope Boniface IV to all Christian martyrs. This took place on May 13, 609 A.D., and this date became a festival, and its scope was extended to include all saints. The date of the festival was later moved to November 1, by Pope Gregory III.
The Roman temple had been turned into a Catholic church. A similar syncretism occurred in the 9th century. Christianity had reached Celtic lands, and little by little Celtic festivals and rites were replaced by Christian ones. November 2 was a date when the Celts honored their dead. Given its closeness to November 1, it was assimilated to Christian festivities and has been known as All Souls Day since the year 1000 A.D.
