In the old historic one-year series one encounters three Sundays prior to Lent that neither belong to Epiphany nor fully to Lent. They were dispensed with when the newer lectionaries came into being, although they are retained in the 'revised' one-year series as found in LSB (I say 'revised' since a comparison with the original historic series will reveal obvious changes in the lections from one to the other.) Having never grown up with these Sundays, and having never observed them in my over two decade ministry, I wondered what their purpose was. Epiphany I understand, and Lent makes sense to me, but why would one need a kind of "pre-Lent" season? It would seem according to one source I found that these Sundays are tied to the need for 40 fast days in the season. At one point in history not all the days of the week were designated as fast days, such as Thursday and Saturday, and of course, Sunday. With two extra days 'off the calendar,' so to speak, one needed to push the season back a bit to pick up the requisite number of fast days.
With the concept of 'fast days' changed in the passing centuries, one wonders what function the old pre-Lent Sundays fulfill. Lent prepares us for the Easter celebration, but does Lent itself need a time of preparation? The Gospel for Septuagesima is from Matthew 20, the parable of the Laborers in the Vineyard. In the three-year series that lection is placed in the latter part of the post-Pentecost Sundays (proper 20). At this point Jesus has left Galilee and is moving toward Jerusalem. Following this parable He openly predicts his suffering and death. Naturally, this would fit a Lenten theme. The following Sunday, Sexagesima, the Gospel is the Parable of the Sower from Luke 8. This lection is omitted from Series C where Luke is normally used. Since this is one of the parables recorded in each of the Gospels, it is not surprising that it would be included instead in Series A, but again in the post-Pentecost season. It seems that the organizers of the three-year series saw these periocopes more in keeping with the second half of the church year, than in the first. This, of course, could be a debatable point.
So, again I come back to my original point: What is the purpose of these three Sundays? My sense is they had a more obvious use in former times when fasting was more prevalent, although one wonders why the season of Lent was simply not extended to include this period. Yet is there still a use for a kind of transition period between post-Epiphany Sundays and the Sundays of Lent? And if so, why? Perhaps someone who knows more of the background of these Sundays might wish to offer an enlightened insight.