As the influence of Christianity spread into Celtic lands, in the 7th century, Pope Boniface IV introduced All Saints' Day, a time to honor saints and martyrs, to replace the pagan festival of the dead. It was observed on May 13. In 834, Gregory III moved All Saint's Day from May 13 to Nov. 1 and for Christians, this became an opportunity for remembering before God all the saints who had died and all the dead in the Christian community. Oct. 31 thus became All Hallows' Eve ('hallow' means 'saint').
Sadly, though, many of the customs survived and were blended in with Christianity. Numerous folk customs connected with the pagan observances for the dead have survived to the present.
In 1517, a monk named Martin Luther honored the faithful saints of the past by choosing All Saints Day (November 1) as the day to publicly charge the Church heirarchy with abandoning biblical faith. This became known as "Reformation Day," a fitting celebration of the restoration the same biblical faith held by the saints throughout church history.