Septuagesima

The three Sundays before Lent are Septuagesima, Sexagesima, and Quinquagesima. Roughly, they are seventy, sixty, and fifty days before Easter Sunday. This year Easter Sunday is on April 12th. If you include today--Septuagesima Sunday--and Easter Sunday in the count, you get 64, 57, and 50 days. According to The Catholic Encyclopedia, Septuagesima is the 9th Sunday before Easter. So the idea of "the 70th day" is a bit of a stretch, but the old name has stuck around. I have a missal from 1966 called The People's Mass Book, which is still in print apparently, in a new edition. It says (p. 32):

Originally, these three pre-Lenten Sundays were celebrated to ask God's protection against enemy armies invading Rome.

Septuagesima is the feminine form of septuagesimus, the Latin ordinal corresponding to LXX, or "seventieth." How about that? WordMan™ was hanging around, looking for work, and the third Sunday before Lent just dropped in his lap.