Taken from   Faith and Practice in the Early Church: Foundations for Contemporary Theology by Carl A. Volz (Minneapolis: Augsburg Publishing House, 1983), p. 123. Carl Volz was given a sabbatical leave by Luther Northwestern Theological Seminary to complete this study. 

         Whereas Easter and Pentecost were observed already in New Testament times, Christmas was not instituted until the fourth century. Its observance was associated with the Epiphany on January 6, when Eastern Christians observed Christ's baptism. Since some Gnostic sets denied that Christ was divine from his birth, and said he was merely adopted into the godhead at his baptism, the West decided a separate festival was needed to emphasize his divinity from the moment of conception, a feast known as the Incarnation of Our Lord.  The earliest date we have for its observation is 336 A. D. in Rome, with Dec. 25 possibly selected by the emperor, Constantine. This may have been chosen because it coincided at that time with the winter soltice, but since the Mithraic cult celebrated that day as the Day of the Unconquered Sun (the days then began to get longer), it may have been chosen to counteract pagan influence. Another theory suggests it was to offer the Christians a substitute for the Roman Saturnalia. In the popular mind the day gradually came to be called Christ's Mass. As for Easter, Christmas was preceded by a period of preparation, the four weeks of Advent. At the end of the fourth century John Chrysostom preached a sermon in which he indicated that Christmas was introduced into his church only ten years earlier, but to his dismay it was too much surrounded with unseemly feasting, drinking, and commercialization. January 6 was retained in the East as the nativity of Christ, but in the West it became Epiphany, that is, the manifestation of Christ as divine. For this reason it was associated with the first revelation of the divinity of Christ, the coming of the Magi, the baptism in the Jordan, and the first miracle at Cana. The Sundays following Epiphany were in effect an extension of the Christmas season, ending with Ash Wednesday and the beginning of Lent.