Father John Hunwicke has a nice piece on Septuagesima, in which he notes,
I incline to believe that S Gregory has left us his own explanation of his liturgical creation, Septuagesima, in the passage from his writings of which the old Breviary gives us a portion in the Third Nocturn (Hom 19 in Evang.; the full text of which is handily available in PL 76 coll 1153sqq.). Speaking, according to the manuscripts, in the basilica of S Lawrence one Septuagesima morning, he explains the different times of the day referred to in the Sunday’s EF Gospel (the parable of the Husbandman hiring labourers for his vineyard): “The morning of the world was from Adam to Noah; the third hour, Noah to Abraham; Sixth, Abraham to Moses; Ninth, Moses to the Lord’s Advent; eleventh, from the Lord’s Advent to the end of the world”.
(Emphasis supplied and hyperlink added.) Read the whole thing there.
One of our very, very few objections to the liturgical reforms of Bl. Paul VI (up there with the four-week psalter) is how smoothly everything seems to hum along, particularly in Tempus Per Annum. We just saw it, in fact. Christmas ends with the Baptism of Our Lord, and then you skate along in Tempus Per Annum until Feria IV Cinerum, and then Lent (and Eastertide and Pentecost). Then, after Pentecost, you’re right back where you left off on Fat Tuesday, skating along until Christ the King and Advent.
The nice thing, then, about Septuagesima, Sexagesima, and Quinquagesima, preserved as they are in the Forma Extraordinaria, is the sense that they create that things might not be so well-oiled as they look in the Forma Ordinaria. The penitential progress toward Easter may not be so squared off, glass smooth, and air conditioned as all that. You’ve got to have your head right to appreciate what is going on. And the Gesimas help with that. As Father Hunwicke notes, “During Lent, of which Septuagesima is the preamble, we repent of the Fall and the mark which it has left on each successive age of human history and on each one of us.” We get the range of the Fall and its destructive effects during the Gesimas, so we can better repent during Lent, and so we can better prepare ourselves for the sorrow of Holy Week and the unrestrained joy of Easter.