Now, the feast of Easter must be prepared for by forty days of recollection 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 distraction 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 for the deficiency, they were obliged to begin their Lent so many days earlier.
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 thirty-six fasting days ... which we offer to God as the tithe of our year.”
Commentary from The Liturgical Year by Dom Propser Guéranger (1805-1875).