All Hallow's Eve

At Halloween, popular histories and certain religious sects (e.g. Jehovah's Witnesses and the Armstrong Churches of God) repeat the well-entrenched theory that this holiday's origins are found in paganism. This theory originated among certain well-meaning historians who sought to tie the practices of the Catholic Church to paganism. Unfortunately, the scholarship and research associated with the resulting "histories" ignored a great deal of the actual history of this holiday's origins (which are well-documented). In other words, the origins of All Hallows' Eve or "Halloween" are found in the history of the Christian faith.

Indeed, Catholic historians tell the story of the holiday this way:

In the early days of the Church, Christians celebrated each martyr’s feast on the anniversary of his death. As the number of martyrs increased over time, though, and because the liturgical calendar can only accommodate a limited number of Christian luminaries in its 365-day cycle, the Church began honoring all martyrs on a single day.

By the later A.D.300s and the early 400s, the Church began to celebrate a feast for all those who had given their life in witness to Christ and his Church. In 609 or 610, Pope Boniface IV consecrated the Pantheon in Rome to the Blessed Virgin Mary and all martyrs, choosing May 13 to honor the martyrs. Pope Gregory III (731-741) extended the feast to include all saints, moving its celebration to November 1, which explains why All Hallows’ Eve is celebrated on October 31, the day before.

See Is Halloween Demonic? The Truth About All Hallows' Eve