Jungmann, Josef A., S.J. "Chapter Twenty-One: The Christmas Cycle." The Early Liturgy to the Time of Gregory the Great. (translated by Francis A. Brunner, C.S.S. R., Notre Dame, Indiana: University of Notre Dame Press, 1959, pp. 266-277.
Jungmann points out the early similarities between the liturgical functions of Christmas and Epiphany, while observing that neither held the importance in the liturgical year which Easter did in the West. The importance of Chrstmas and Epiphany increased in the liturgy during the fifth century (Jungmann 1959, 266). The Christmas observances were "nocturnal" services, generally beginning in Jerusalem with a pilgrmage to Bethlehem, a mass, then a return at dawn to Jerusalem. This was followed by a rest and another mass (Jungmann 1959, 267). The pattern, though without the town of Bethlehem, was adopted in other locations. By the time of Gregory the Great a third mass was commonly used, at least in the papal chapel and probably elsewhere (Jungmann 1959, 268).
The development of extended days of holiday celebratin, according to Jungmann, follows a predictable pattern. The celebration first exists on a day, then is observed additionally in some manner a week later. Then it may grow to the "after-celebration" we now see (Jungmann 1959, 269). Examples are Easter with its 50 days and Christmas with twelve days, filling the calendar until the next holiday. Because of this phenomenon, lesser observances are sometimes subsumed into the greater one, with the liturgy and lectionaries blending.
Because feasts tend to lengthen as "after-celebrations" one must ask where Advent came about. Jungmann finds a multiple-week recognition in Gaul in the fourth century (Jungmann 1959, 271). He theorizes that it was related to the so-called "ember days" of December, which were early times of dedication to prayer and fasting for a week in June, September, and December. A prayer vigil was kept the Saturday night of the ember week, taking the place of the regular Sunday service.
The Gallic Advent observance has an earlier and clearer origin than that of the East (Jungmann 1959, 274). By the hear 490 there is a feast which starts November 11 and continues to Christmas. It likely was an actual preparation for Epiphany, celebrated in Gaul before Christmas was a normal event on the calendar. The parallel to the forty day preparation fo Easter is hard to mistake.