CHRISTMAS (from Old English Cristes maesse “Christ’s mass”).† Observance commemorating the birth of Jesus. In the Western church, the Feast of the Nativity of Our Lord was first celebrated on December 25 ca. A.D. 336, the date apparently chosen to counter the Roman feast Natalis Solis Invicti (“birth of the unconquered sun”), the birthday of Emperor Aurelius. In Alexandria and the Eastern churches the event was originally celebrated on January 6 in connection with the Feast of the Epiphany honoring Jesus’ baptism; some branches of the Eastern church still hold to this date. The name, which does not occur in the New Testament, derives from the three masses of the Western rite celebrating the threefold birth of Christ in the Father’s bosom, Mary’s womb, and the believer’s soul.
—Allen C. Myers, The Eerdmans Bible Dictionary (Grand Rapids, MI: Eerdmans, 1987), 210–211.