No one is quite sure why Septuagesima Sunday bears that name. Literally, Septuagesima means "seventieth" in Latin, but contrary to common error, it is not 70 days before Easter, but only 63. The most likely explanation is that Septuagesima Sunday and Sexagesima Sunday simply derived their names from Quinqagesima Sunday, which is 49 days before Easter, or 50 if you include Easter. (Quinqagesima means "fiftieth.")
In any case, it was common for early Christians to begin the Lenten fast immediately after Septuagesima Sunday. Just as Lent today begins 46 days before Easter, since Sundays are never a day of fasting (see "How Are the 40 Days of Lent Calculated?"), so, in the early Church, Saturdays and Thursdays were considered fast-free days. In order to fit in 40 days of fasting before Easter, therefore, the fast had to start two weeks earlier than today.
In the celebration of the traditional Latin Mass, starting on Septuagesima Sunday, neither the Alleluia nor the Gloria are sung. (See "Why Don't Roman Catholics Sing the Alleluia During Lent?") They do not return until the Easter Vigil, when we mark the triumph of Christ over death in His Resurrection.