The season of Septuagesima comprises the three weeks immediately preceding Lent.
From the 1957 St. Joseph’s Dail Missal
“With Septuagesima begins the Pre-Lenten season which comprises Septuagesima, Sexagesima, and Quinquagesima Sundays. These three weeks form a transition from the joy of Christmastide to the austerity of the Penitential Season of Lent. This is the season when the Church most earnestly invites her children to prepare properly for the Feast of Christ’s glorious Resurrection. She urges them to rise from and expiate their sins; to practice prayer and charity with greater zeal; to become indeed new men through God’s grace.”
Purple vestments are worn to signify Pennance during this period except Laetare Sunday on witch Rose Color Vestments are used. Also the Alleluia and Gloria are omitted during Masses from Septuagesima Sunday to Holy Saturday.