With the spread of Christianity in the Roman Empire, attempts were made to blend or replace previous pagan festivals with Christian ones. This was part of an overall strategy to make the new religion more acceptable to traditional societies.
In the 7th century, Pope Boniface IV introduced All Saints' Day, originally on May 13, to honor all saints and martyrs. This celebration was a day to honor those who had reached the glory of heaven. The day was dedicated to "all the saints" and was intended to replace the pagan festival of Lemuria.
In the 8th century, Pope Gregory III moved All Saints' Day to November 1. The night before became known as All Hallows Eve, which eventually was shortened to Halloween. This could have been an attempt to replace or assimilate the Celtic festival of Samhain with a Christian celebration. The Church maintained the celebrations of bonfires, parades, and costumes, showing a blending of Christian and pagan traditions.
Later on, in the 11th century, the church would make November 2 All Souls' Day, a day to honor and pray for the souls of the dead in purgatory. The tradition of All Souls' Day was initiated by Abbot Odilo of Cluny at his French monastery in 998 A.D. and was quickly adopted across Europe. Like Samhain and All Saints' Day, All Souls' Day was celebrated with large bonfires, parades, and costumes. This trio of celebrations: All Hallows Eve (Halloween), All Saints Day, and All Souls Day, are collectively known as Hallowmas.
These changes helped ensure the survival of some of the traditions and celebrations of Samhain, which continue to be associated with Halloween to this day.