From Dom Guéranger's The Liturgical Year.
The season of Septuagesima comprises the three weeks immediately preceding Lent. It forms one of the principal divisions of the liturgical year, and is itself divided into three parts, each part corresponding to a week: the first is called Septuagesima; the second, Sexagesima; the third, Quinquagesima.
All three are named from their numerical reference to Lent, which, in the language of the Church, is called Quadragesima, that is, Forty, because the great feast of Easter is prepared for by the holy exercises of forty days. The words Quinquagesima, Sexagesima, and Septuagesima, tell us of the same great solemnity as looming in the distance, and as being the great object towards which the Church would have us now begin to turn all our thoughts, and desires, and devotion.
Now, the feast of Easter must be prepared for by forty days of recollectedness and penance. Those forty days are one of the principal seasons of the liturgical year, and one of the most powerful means employed by the Church for exciting in the hearts of her children the spirit of their Christian vocation. It is of the utmost importance that such a season of grace should produce its work in our souls—the renovation of the whole spiritual life. The Church, therefore, has instituted a preparation for the holy time of Lent. She gives us the three weeks of Septuagesima, during which she withdraws us, as much as may be, from the noisy distractions of the world, in order that our hearts may be more readily impressed by the solemn warning she is to give us at the commencement of Lent by marking our foreheads with ashes.
This prelude to the holy season of Lent was not known in the early ages of Christianity: its institution would seem to have originated in the Greek Church. Besides the six Sundays of Lent, on which by universal custom the faithful never fasted, the practice of this Church prohibited fasting on the Saturdays likewise; consequently their Lent was short by twelve days of the forty spent by our Saviour doing penance in the desert. To make up the deficiency, they were obliged to begin their Lent so many days earlier, as we will show in our next volume.
The Church of Rome had no such motive for anticipating the season of those privations which belong to Lent; for, from the earliest antiquity, she kept the Saturdays in Lent (and as often during the rest of the year as circumstances might require) as fasting days. At the close of the sixth century, St. Gregory the Great alludes, in one of his homilies, to the fast of Lent being less than forty days, owing to the Sundays which come during that holy season. 'There are,' he says, ‘from this day (the first Sunday of Lent) to the joyous feast of Easter, six weeks, that is, forty-two days. As we do not fast on the six Sundays, there are but thirtysix fasting days . . . which we offer to God as the tithe of our year.’
It was, therefore, after the pontificate of St Gregory, that the last four days of Quinquagesima week were added to Lent, in order that the number of fasting days might be exactly forty. As early, however, as the ninth century, the custom of beginning Lent on Ash Wednesday was of obligation in the whole Latin Church. All the manuscript copies of the Gregorian Sacramentary, which bear that date, entitle this Wednesday In capite jejunii, that is to say, the beginning of the fast; and Amalarius, who gives us every detail of the liturgy of the ninth century, tells us that it was, even then, the rule to begin the fast four days before the first Sunday of Lent. We find the practice confirmed by two Councils, held in that century. But, out of respect for the form of divine service drawn up by St. Gregory, the Church does not make any important change in the Office of these four days. Up to the Vespers of Saturday, when alone she begins the lenten rite, she observes the rubrics prescribed for Quinquagesima week.
Peter of Blois, who lived in the twelfth century, tells us what was the practice in his days. He says: ‘All religious begin the fast of Lent at Septuagesima; the Greeks, at Sexagesima; the clergy, at Quinquagesima; and the rest of Christians, who form the Church militant on earth, begin their Lent on the Wednesday following Quinquagesima.’ The secular clergy, as we learn from these words, were bound to begin the lenten fast somewhat before the laity; though it was only by two days—that is, on Monday, as we gather from the Life of St. Ulric, bishop of Augsburg, written in the tenth century. The Council of Clermont, in 1095, at which Pope Urban II. presided, has a decree sanctioning the obligation of the clergy to begin abstinence from flesh meat at Quinquagesima. This Sunday was called, indeed, Dominica carnis privii, and carins privium sacerdotum, that is, priests’ carnival Sunday; but the term is to be understood in the sense of the announcement being made, on that Sunday, of the abstinence having to begin on the following day. We shall find, further on, that a like usage was observed in the Greek Church on the three Sundays preceding Lent. This law, which obliged the clergy to these two additional days of abstinence, was in force in the thirteenth century, as we learn from a Council held at Angers, which threatens with suspension all priests who neglect to begin Lent on the Monday of Quinquagesima week.
This usage, however, soon became obsolete; and in the fifteenth century, the secular clergy, and even the monks themselves, began the lenten fast, like the rest of the faithful, on Ash Wednesday.
There can be no doubt that the original motive for this anticipation—which, after several modifications, was limited to the four days immediately preceding Lent—was to remove from the Greeks the pretext of taking scandal at the Latins, who did not fast fully forty days. Ratramnus, in his Controversy with the Greeks, clearly implies it. But the Latin Church did not think it necessary to carry her condescension farther, by imitating the Greek ante-lenten usages, which originated, as we have already said, in the eastern custom of not fasting on Saturdays.
Thus it was that the Roman Church, by this anticipation of Lent by four days, gave the exact number of forty days to the holy season, which she had instituted in imitation of the forty days spent by our Saviour in the desert. Whilst faithful to her ancient practice of looking on the Saturday as a day appropriate for penitential exercises, she gladly borrowed from the Greek Church the custom of preparing for Lent, by giving to the liturgy of the three preceding weeks a tone of holy mournfulness. Even as early as the beginning of the ninth century, as we learn from Amalarius, the Alleluia and Gloria in excelsis were suspended in the Septuagesima Offices. The monks conformed to the custom, although the Rule of St. Benedict prescribed otherwise. Finally, in the second half of the eleventh century, Pope Alexander II. enacted that the total suspension of the Alleluia should be everywhere observed, beginning with the Vespers of the Saturday preceding Septuagesima Sunday.
This Pope was but renewing a rule already sanctioned, in that same century, by Pope Leo IX., and inserted in the body of Canon Law.
Thus was the present important period of the liturgical year, after various changes, established in the cycle of the Church. It has been there upwards of a thousand years. Its name, Septuagesima (seventy), expresses, as we have already remarked, a numerical relation to Quadragesima (the forty days); although, in reality, there are not seventy but only sixty-three days from Septuagesima Sunday to Easter. We will speak of the mystery of the name in the following chapter. The first Sunday of Lent being called Quadragesima (forty), each of the three previous Sundays has a name expressive of an additional ten; the nearest to Lent, Quinquagesima (fifty); the middle one, Sexagesima (sixty); the third, Septuagesima (seventy).
As the season of Septuagesima depends upon the time of the Easter celebration, it comes sooner or later according to the changes of that great feast. January 18 and February 22 are called the ‘Septuagesima keys,' because the Sunday, which is called Septuagesima, cannot be earlier in the year than the first, nor later than the second, of these two days.
 The sixteenth Homily on the Gospels.
 Meaux and Soissons.
 Sermon xiii.
 The Gallican liturgy had retained several usages of the oriental Churches, to which it owed, in part, its origin: hence, it was not without some difficulty that the custom of fasting and abstaining on Saturdays was introduced into Gaul. Until such time as the Churches of that country had adopted the Roman custom, in that point of discipline, they were necessitated to anticipate the fast of Lent. The first Council of Orleans, held in the early part of the sixth century, enjoins the faithful to observe, before Easter, Quadragesima (as the Latins call Lent), and not Quinquagesima, 'in order,' says the Council, 'that unity of custom may be maintained.’ Towards the close of the same century, the fourth Council held in the same city, repeats the same prohibition, and explains the intentions of making such an enactment, by ordering that the Saturdays during Lent should be observed as days of fasting. Previously to this, that is, in the years 511 and 541, the first and second Councils of Orange had combated the same abuse, by also withdrawing from the faithful the obligation of commencing the fast at Quinquagesima. The introduction of the Roman liturgy into France, which was brought about by the zeal of Pepin and Charlemagne, finally established in that country the custom of keeping the Saturday as a day of penance; and as we have just seen, the beginning Lent on Quinquagesima was not observed excepting by the clergy. In the thirteenth century, the only Church in the patriarchate of the west, which began Lent earlier than the Church of Rome, was that of Poland: its Lent opened on the Monday of Septuagesima, which was owing to the rites of the Greek Church being so much used in Poland. The custom was abolished, even for that country, by Pope Innocent IV. in the year 1248.
 Cap. Hi duo. De consec. Dist. 1