However, while this is true for many of us, it is not so for all of us. The key is that we are speaking here of the Roman calendar, and this time of the year provides a very visible opportunity to remind ourselves of the fact that there are other expressions within the Western liturgical tradition. For instance, within the calendar of the Ambrosian rite, this past weekend actually marked their First Sunday of Advent. (See right. Image courtesy of Ambrosianeum) This is for the reason that the Ambrosian rite observes, not four Sundays of Advent as we observe within the Roman calendar, but rather six Sundays of Advent.The Ambrosian rite is not alone in this, for likewise within the Mozarabic rite are six Sundays of Advent observed. This stands not only in the case of the more ancient forms of their respective liturgical books, but also in the post-conciliar editions as well I would note.
In fact, if you look historically at the season of Advent you will see various numbers mentioned in relation to the duration of its observance, from three, four, five to six Sundays of Advent, depending on time and place. (With three being debated.)
As a supplement, I would like to add this brief selection of quotations, primarily from Archdale King, which speaks to some of the present day variation, as well as to some of the historical variations. I would make note that, as with so much in the way of historical inquiry, not all scholars will agree on all the historical particulars. Accordingly, the intent in providing these is simply to lend a further sense to the historical aspects and questions. (Others may wish to add further quotations from other sources in the comments.)
Regarding the Roman rite:
"The observance of the season of Advent is found first in Spain and Gaul, and there is no trace of it at Rome until after the time of Leo the Great (ob. 461)...The number of Sundays at first varied, and the ancient Roman documents indicate six... A pre-Gregorian document shows five Sundays, but from the time of St. Gregory (ob. 604) the number had been reduced to four..." (Archdale King, Liturgy of the Roman Church, p. 186)
"For many centuries the Roman Church has set aside four weeks for the keeping of Advent. It is true that the Gelasian and Gregorian Sacramentaries, as well as several other ancient lectionaries, reckon five weeks, but the lectionary lists of Capua and Naples, and the custom of the Nestorians... know only four weeks of Advent..." (Ildefonso Schuster, The Sacramentary, vol. 1, p. 320)
Regarding the Gallican rite:
"The key-day for Advent Sunday was fixed by the first council of Macon (581) for the feast of St. Martin (11 November), so that, as in the Ambrosian and Mozarabic liturgies, we find six Sundays in Advent. The Gallicanum Vetus provides two Sunday Masses. An alleged work of St. Hilary of Poitiers (ob. c. 368), quoted by Migne and ascribed to Berno of Rechenau (ob. 1048), gives Advent no more than three Sundays: tres tantum hebdomadae in adventu Domini It is, however, impossible to attribute a three weeks' Advent either to St. Hilary or to the 4th century." (Archdale King, Liturgies of the Past, p. 133-4)
Regarding the Ambrosian rite:
"Milan has observed six weeks of Advent from at least the 7th century..." (Archdale King, Liturgies of the Primatial Sees, p. 331)
"The year begins with the First Sunday of Advent, but as in the Mozarabic rite, two weeks earlier than Rome, thus providing six Sundays. The key date for the sanctoral is 11 November, the feast of St. Martin: Adventus Domini inchoatur Dominica post festum Sancti Martini, that is a day between 12 and 18 November." (Ibid., p. 331)
Regarding the Mozarabic rite:
"It is the common opinion of writers that the season of Advent was adopted in Spain in the 5th century... There were normally six Sundays, as in Gaul and Milan, with the key-day 11 November, although some of the MSS. give but five." (Ibid., p. 534)
