Christmas (Christ's Mass), annual Christian festival observed on Dec. 25 to commemorate the birth of Jesus Christ. The exact date of Christ's birth is unknown. The origin of the Christmas story is based on the Gospels of Luke and Matthew in the New Testament. The first reference to the celebration of Christmas occurred in a Roman calendar of A.D. 336. By the Middle Ages, Christmas had become the most significant religious holiday in Europe, with Saint Nicholas as a symbol of giving. In the 19th century Christmas trees and Christmas cards became customs of the holiday. In the United States, Santa Claus replaced Saint Nicholas as the great gift giver.