This period of liturgical time is probably that which shows forth the single most noticeable variance between the two Roman calendars because of their respective liturgical character and characteristics.
For Catholics who have only or primarily known the modern form of the Roman calendar -- and perhaps even for some of those who worship within the context of the calendar of the usus antiquior -- it no doubt strikes one as a unique element of the older Roman calendar, and it may invite the questions, "what is it and why is it done?"
Pre-Lent within the Byzantine Liturgical Calendar
While it might seem unique to the older Roman calendar, it is worth noting that within the Byzantine liturgical calendar, they too celebrate a pre-Lenten period of similar duration. Accordingly, the pre-Lenten period is a point of unity between the usus antiquior and the Byzantine liturgy.
Within the Byzantine liturgical calendar, their pre-Lent begins with the Sunday of the Publican and the Pharisee (which constitutes their "70th"), continues through the Sunday of the Prodigal Son, followed by what is popularly known as Meatfare Sunday (after which fasting from meat begins), and finally Cheesefare Sunday (after which fasting from dairy products is observed). Great Lent then begins.
The Origins and Purpose of Pre-Lent
With regard to the origins of the pre-Lenten period, many liturgical writers attribute the beginnings of pre-Lent to the desire to accomplish the 40 days fast -- since there were non-fast days within the weeks of Lent proper which accordingly did not amount to 40 days of fasting. Through piety and devotion, this was extended further still. The specific time of origin is not agreed upon, but various dates surrounding the time of St. Gregory the Great in the 6th and 7th centuries are noted in various respects -- with the devotional extension of the fast being mentioned even earlier. According to Duchesne, the fourth Council of Orleans mentions Quinqagesima and Sexagesima around A.D. 541 -- albeit it by way of disapproval.
The purpose of pre-Lent seems to be the same in both the Byzantine East and more ancient form of the Roman rite; it is a period of progressive preparation and movement toward Lent and ultimately Easter.
Fr. Weiser, in his Handbook of Christian Feasts and Customs speaks of it accordingly:
The liturgical preparation for the greatest feast of Christianity -- Easter -- proceeds in five periods of penitential character. As the observance of this preparation apporaches the feast, the penitential note grows progressively deeper and stricter. The first period of this season of pre-Lent, from Septuagesima Sunday to Ash Wednesday; the second extends from Ash Wednesday to Passion Sunday; the third comprises Passion Week; the fourth includes the days of Holy Week up to Wednesday; the fifth consists in the Sacred Triduum (Holy Thursday, Good Friday, and Holy Saturday.) In these three days, which are devoted entirely to the commemoration of the Lord's Passion, the penitential observance reaches its peak, until it ends (at the Easter Vigil) in the glorious and joyful celebration of the Resurrection. (p. 154-5)
How do we see this manifest then? Within the Byzantine rite we see the progressive movement from non-fasting to the gradual tightening of the discipline beginning on Meatfare Sunday leading through until the Great Fast itself. Within the Roman rite, we see the penitential character which the liturgical rites take on during pre-Lent, before finally proceeding into Lent itself with its fast -- though a note must be added at this point: since 1966, this has now taken the form of being an optional devotional fast as well, but for Ash Wednesday, Good Friday and all Fridays generally within Lent when fasting yet remains obligatory; see the 1966 Apostolic Constitution of Pope Paul VI, Paenitemini.