Halloween had its beginnings in an ancient, pre-Christian Celtic festival of the dead called Samhain. Samhain became the Halloween we are familiar with when Christian missionaries attempted to change the religious practices of the Celtic people by assigning the Christian feast of All Saints or All Hallows to November 1 and All Souls Day to November 2. These feast days, in addition to serving important functions in the Christian calendar, were intended to substitute for Samhain and ultimately to replace it. Instead, elements of Christian and pre-Christian belief mingled, creating the popular festival of Halloween, or All Hallows Eve--the festival we still observe today.
Winick, Stephen. “Research Guides: Halloween & Día de Muertos Resources: Introduction.” Research guide. Accessed October 24, 2024. https://guides.loc.gov/halloween/introduction.