Thomas J. Talley in The Origins of the Liturgical Year (Pueblo Publishing Company) sees the beginning of an advent season in the Fourth Canon of the Council of Saragosa in 380. In 567, the Synod of Tours established a December fast. And in 581 the Council of Macon ordered an advent fast for the laity from the Feast of St. Martin (November 11) to Christmas. This took the name of St. Martin's Lent.
In the seventh and eighth centuries, lectionaries (books containing the scriptural readings for the Liturgy of the Word) provided for six Sundays in Advent.
According to the Harper Collins Encyclopedia of Catholicism, edited by Richard P. McBrien, Gregory the Great, who died in 604, was the real architect of the Roman Advent. Gregory fixed the season at four weeks and composed seasonal prayers and antiphons. Gaul (France) enriched the season with eschatological elements. And the fusion of the Roman and Gallican observances returned to Rome by the 12th century.