The third Sunday before Lent. Septuagesima is also the name given to these three weeks before Ash Wednesday. The next two Sundays in this period are called Sexagesima and Quinquagesima. The Sunday after Ash Wednesday is called Quadragesima.
As related in the Golden Legend: "The time of Septuagesima representeth the time of deviation, that is of transgression. The Sexagesima signifieth the time of revocation. The Quinquagesima signifieth the time of remission. The Quadragesima signifieth of penance and satisfaction."
This is our preparation for Lent. It is time to consider our sins and all which keeps us separate from God; during Lent, we will mourn our sins and repent, mortifying ourselves with fasting, abstinence and prayers.
Read an overview of the season of Septuagesima, its meaning and customs, here, and here.
The Alleluia is removed from our liturgy, and will not be said again until Easter. There were different ways to represent the 'death' of the Alleluia, the most well-known being the ceremony of its burial. From Curiosities of Popular Customs: "It was formerly distinguished in England by a strange ceremony denominated the Funeral of Alleluia. On the Saturday of Septuagesima, at nones, the choristers assembled in the great vestiary of the cathedral, and there arranged the ceremony. Having finished the last benedicamus, they advanced with crosses, torches, holy water, and incense, carrying a turf in the manner of a coffin, passed through the choir, and went howling to the cloister as far as the place of interment; and then, having sprinkled the water and censed the place, they returned by the same road." (fosbroke: British Monachism, 1843, p. 56.)