October 31st, the eve of the Western Christian feast of All Hallow’s Day. It initiates the triduum of Hallowmas, the time in the liturgical year dedicated to remembering the dead, including saints (hallows), martyrs, and all the faithful departed believers.
All Hallows’ Eve is a Christianized feast initially influenced by Celtic harvest festivals, with possible pagan roots, particularly the Gaelic Samhain. Other academic scholars maintain that it originated independently of Samhain and has solely Christian roots. According to a Christian source, Halloween has Christian roots and pagan roots.
Celebrated on November 1st by the Catholic Church, and on the first Sunday after Pentecost in Eastern Catholicism, in honor of all the saints, known and unknown. All Saints’ Day is the second day of Hallowmas, and begins at sunrise on the first day of November and finishes at sundown. It is the day before All Soul’s Day.