1st Vespers of Septuagesima Sunday

Benedicamus Domino, Alleluia, Alleluia!
Deo Gratias, Alleluia, Alleluia!

Today the Liturgy says farewell to "Alleluia" until Holy Saturday.

The Inn had this to say a decade ago:

The medieval Church had several ceremonies of Farewell to the Alleluia which took place on the eve of Septuagesima. From Dom Gueranger:  

The farewell to the Alleluia, in the Middle Ages, varied in the different Churches. Here, it was an affectionate enthusiasm, speaking the beauty of the celestial word; there, it was a heart-felt regret at the departure of the much-loved companion of all their prayers.  

We begin with two antiphons, which would seem to be of Roman origin. We find them in the Antiphonarium of Saint Cornelius of Compiegne, published by Dom Denys de Sainte Marthe. They are a farewell to Alleluia made by our Catholic forefathers in the ninth century; they express, too, the hope of its coming back, as soon as the Resurrection of Jesus shall have brightened up the firmament of the Church.  

ANT. Angelus Domini bonus comitetur tecum, Alleluia, et bene disponat itineri tuo, ut iterum cum gaudio revertaris ad nos, Alleluia, Alleluia.
ANT. May the good angel of the Lord accompany thee, Alleluia, and give thee a good journey, that thou mayst come back to us in joy, Alleluia.  

ANT. Alleluia, mane apud nos hodie, et crastina proficisceris, Alleluia ; et dum ortus fuerit dies, ambulabis vias tuas, Alleluia, Alleluia, Alleluia.
ANT. Alleluia, abide with us today, and tomorrow thou shalt set forth, Alleluia ; and when the day shall have risen, thou shalt proceed on thy way, Alleluia, Alleluia, Alleluuia.


From the Gothic Church of Spain, an anthem: 

Ibis, Alleluia. Prosperum iter habebis Alleluia; et iterum cum gaudio revertaris ad nos, Alleluia. In manibus enim suis portabunt te: ne unquam offendas ad lapidem pedem tuum. Et iterum cum gaudio revertaris ad nos, Alleluia.
Thou shalt go, Alleluia; thy journey shall be prosperous, Alleluia; and again come back to us with joy, Alleluia. For they shall bear thee up in their hands, lest at any time thou dash thy foot against a stone. And again come back to us with joy, Alleluia. 

There's more here (although some of the links therein, alas, have died the death.)