The modern observance of Advent in the Roman Catholic Church focuses on the four weeks that precede the celebration of Christmas.
Originally, the celebration of Advent was similar to that of Lent. Starting in the 5th century, Advent was a period of 40 penitential days that began on the feast of St. Martin on November 11 or 12.
This fast was often called “St. Martin’s Fast” or “St. Martin’s Lent.”
In the 6th century, St. Gregory the Great trimmed down the season of Advent to include only the four weeks that preceded Christmas (though some historical sources indicate it may have included five Sundays).
Dom Prosper Gueranger summarizes this tradition in his Liturgical Year:
The liturgical form of Advent as it now exists in the Roman Church has gone through certain modifications. St. Gregory seems to have been the first to draw up the Office for this season, which originally included five Sundays as is evident from the most ancient sacramentaries of this great Pope. It even appears probable ... that St. Gregory originated the ecclesiastical precept of Advent, although the custom of devoting a longer or shorter period to a preparation for Christmas has been observed from time immemorial.
The New Catholic Encyclopedia claims that, "[St. Gregory] shortened the season from 6 weeks to 4."
It appears that St. Gregory may be one of the main reasons why Catholics in the Roman Rite celebrate a four-week Advent, and why some historians call it "Gregory's Advent."
