As Amy Welborn notes on her blog, the suppression of the Septuagesima season after the Second Vatican Council was unfortunate for many reasons, including ecumenical ones--after all, the Church of England observes it, some high church Lutherans observe it, and Eastern Rite Catholics observe a pre-Lenten season too. Within the Roman Rite, the Anglican Use of the Ordinariate (thus the poster from St. Luke's Ordinariate in Washington, DC) and the Extraordinary Form still observe Septuagesima.
Welborn links an article explaining the development and purpose of Septuagesima and why it was suppressed--Welborn summarizes the latter: "As usual, it was determined that all this was too hard for us."
The intuition of faith that gave rise to the season of Septuagesima, then, is a recognition of the goodness, or even the necessity, of preparing oneself mentally, physically, and spiritually for the ascesis of Lent. The same recognition expresses itself in the Eastern practice of observing the Sundays of Meatfare and Cheesefare on the second and first Sundays, respectively, that precede the last Sunday before Lent. From Meatfare Sunday the faithful lay aside all flesh foods until the Pasch. From Cheesefare Sunday the faithful abstain from animal products such as eggs, milk, yogurt, butter, and cheese.
Meatfare and Cheesefare Sunday are practiced and well understood in Eastern Rite Catholic and Eastern Orthodox communities, but the committee reviewing Septuagesima thought it was too hard for Western Roman Rite Catholics:
"The penitential character of the time of Septuagesima or pre-Lent (the suppression of the Gloria and Alleluia, the color violet) is difficult for the faithful to understand without many explanations. At present the exterior signs of a penitential season are used as in Lent, but are not retained for the sake of particular penance as in Lent. According to the mind of the Constitution on the Sacred Liturgy, the arrangement of the liturgical year may become clearer through the suppression of the penitential character of this season. As it pertains to the names Septuagesima, Sexagesima, and Quinquagesima: because these names are also used by some of our separated brethren, it is necessary to proceed cautiously in this matter and to promote conversations on this matter as on others."
If you go to Welborn's blog post she gives examples of easily understood catechesis for 7th graders on Septuagesima's preparation for Lent.
I'll be citing Parochial and Plain Sermons by Newman from all three Sundays next Wednesday and really look forward to saying their names: Septuagesima, Sexagesima, and QUINQUAGESIMA!