I have here the Pictorial Bible Dictionary's definition of Christmas:
Christmas: the anniversary of the birth of Jesus Christ, and its observance, celebrated by most Protestants and by Roman Catholics on Dec. 25; by Eastern Orthodox churches on Jan. 6; and by the Armenian church on Jan. 19. The first mention mention of its observance on Dec. 25 is in the time of Constantine, about A.D. 325. The date of the birth of Christ is not known. The word Christmas is formed of Christ + Mass, meaning a mass of religious service in commemoration of the birth of Christ. Whether the early Christians thought of or observed Christmas is not clear. Once introduced, the observance spread throughout Christendom. Some Christian bodies disapprove of the festival. Many customs of pagan origin have become part of Christmas, e.g. the Christmas tree; but most of these no longer have a heathen connotation, but have acquired a Christian meaning (e.g., the Christmas tree points upward to God and reminds us of His gifts). A commemoration of the birth of Christ in harmonious keeping with the events surrounding the birth (Luke 2:1-20; Matt. 1:18-2:12) is a natural and normal expression of love and reverence fro Jesus Christ.