In the 7th century, Pope Boniface IV established a day of feasting to honor all Christian martyrs on May 13, known as All Martyr's Day. Then, in the 8th century, November 1 was declared by Pope Gregory III as All Saint's Day, a holiday to honor all saints and martyrs. All Hallow's Eve took place the night before and involved a vigil held by the Catholic Church in which worshippers prepared for the next day's festival by fasting and praying.
It is believed that All Saint's Day was an attempt to replace the ancient Celtic festival of Samhain with a church-sanctioned holiday as Christianity spread into Celtic lands in the 9th century. By observing a festival at this time of year, Christian missionaries hoped to create a smooth transition from existing Pagan practices to those of Christianity. For this reason, many of the traditions and events celebrated on All Saint's Day mirror those of Samhain.
Samhain, meaning "end of summer" in Gaelic, was a celebration of the end of the harvest season, in preparation for the coming of winter. It involved large bonfires and people dressed in costume for the purpose of warding off roaming ghosts and spirits. All Saint's Day was celebrated in the same way, with bonfires, parades, and people dressed in costume as saints, angels and devils.