The Catholic Encyclopedia
(Latin septuagesima, the seventieth).
Septuagesima is the ninth Sunday before Easter, the third before Lent known among the Greeks as "Sunday of the Prodigal" from the Gospel, Luke 15, which they read on this day, called also Dominica Circumdederunt by the Latins, from the first word of the Introit of the Mass. In liturgical literature the name "Septuagesima" occurs for the first time in the Gelasian Sacramentary. Why the day (or the week, or the period) has the name Septuagesima, and the next Sunday Sexagesima, etc., is a matter of dispute among writers. It is certainly not the seventieth day before Easter, still less is the next Sunday the sixtieth, fiftieth, etc. Amularius, "De eccl. Off.", I, I, would make the Septuagesima mystically represent the Babylonian Captivity of seventy years, would have it begin with this Sunday on which the Sacramentaries and Antiphonaries give the Introit "Circumdederunt me undique" and end with the Saturday after Easter, when the Church sings "Eduxit Dominus populum suum." Perhaps the word is only one of a numerical series: Quadragesima, Quinquagesima, etc. Again, it may simply denote the earliest day on which some Christians began the forty days of Lent, excluding Thursday, Saturday, and Sunday from the observance of the fast.
Septuagesima is today inaugurated in the Roman Martyrology by the words: "Septuagesima Sunday, on which the canticle of the Lord, Alleluja, ceases to be said". On the Saturday preceding, the Roman Breviary notes that after the "Benedicamus" of Vespers two Alleluias are to be added, that thenceforth it is to be omitted till Easter, and in its place "Laus tibi Domine" is to be said at the beginning of the Office. Formerly the farewell to the Alleluia was quite solemn. In an Antiphonary of the Church of St. Cornelius at Compiègne we find two special antiphons. Spain had a short Office consisting of a hymn, chapter, antiphon, and sequence. Missals in Germany up to the fifteenth century had a beautiful sequence. In French churches they sang the hymn "Alleluia, dulce carmen" (Guéranger, IV, 14) which was well-known among the Anglo-Saxons (Rock, IV, 69). The "Te Deum" is not recited at Matins, except on feasts. The lessons of the first Nocturn are taken from Genesis, relating the fall and subsequent misery of man and thus giving a fit preparation for the Lenten season. In the Mass of Sunday and ferias the Gloria in Excelsis is entirely omitted. In all Masses a Tract is added to the Gradual.
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Obstat, Nihil. Lafort, Remy, Censor. Entry for 'Septuagesima'. The Catholic Encyclopedia. https://www.studylight.org/encyclopedias/eng/tce/s/septuagesima.html. Robert Appleton Company. New York. 1914.