What is the origin of Halloween?
In AD 835, Pope Gregory IV designated November 1 as All Saints’ Day, or All Hallows’ Day (the term hallow refers to saints). The night before November 1, October 31, was known as All Hallows’ Evening. How did we get the term Halloween? Look at the name “All Hallows’ Evening.” If we drop the word “all,” the “s” on Hallows’, and the “v” and “ing” on evening, the result spells Halloween.
Long before the church gave this name to the evening before All Saints’ Day (a celebration in remembrance of saints and martyred saints), it had been celebrated in various ways in many places around the world. The book Every Day’s a Holiday accurately observes that Halloween “probably combines more folk customs the world around than will ever be sorted out, catalogued and traced to their sources.” - JOHN ANKERBERG