We must look upon Advent in two different lights: first, as a time of preparation, properly so called, for the birth of our Saviour, by works of penance; and secondly, as a series of ecclesiastical Offices drawn up for the same purpose. We find, as far back as the fifth century, the custom of giving exhortations to the people in order to prepare them for the feast of Christmas. We have two sermons of Saint Maximus of Turin on this subject, not to speak of several others which were formerly attributed to St. Ambrose and St. Augustine, but which were probably written by St. Cesarius of Aries. If these documents do not tell us what was the duration and what the exercises of this holy season, they at least show us how ancient was the practice of distinguishing the time of Advent by special sermons. Saint Ivo of Chartres, St. Bernard, and several other doctors of the eleventh and twelfth centuries, have left us set sermons de Adventu Domini, quite distinct from their Sunday homilies on the Gospels of that season. In the capitularia of Charles the Bald, in 846, the bishops admonish that prince not to call them away from their Churches during Lent or Advent, under pretext of affairs of the State or the necessities of war, seeing that they have special duties to fulfill, and particularly that of preaching during those sacred times.
The oldest document in which we find the length and exercises of Advent mentioned with anything like clearness, is a passage in the second book of the History of the Franks by St. Gregory of Tours, where he says that St. Perpetuus, one of his predecessors, who held that see about the year 480, had decreed a fast three times a week, from the feast of St. Martin until Christmas. It would be impossible to decide whether St. Perpetuus, by his regulations, established a new custom, or merely enforced an already existing law. Let us, however, note this interval of forty, or rather of forty-three days, so expressly mentioned, and consecrated to penance, as though it were a second Lent, though less strict and severe than that which precedes Easter.~from Dom Gueranger's The Liturgical Year, Vol. I