The Tenebrae used to be sung (or said) on each day of the Triduum, either in the early morning hours before sunrise or late on the previous evening. The texts were drawn from the epistles of St. Paul, the writings of St. Augustine, and, most famously, the Biblical book of Lamentations (attributed to Jeremiah). The Tenebrae service was marked especially by the gradual extinguishing of candles in the church as the service proceeded, giving it a wonderfully dramatic structure. The observance of this office was, I understand, deleted in the liturgical reforms of Vatican II, so that it is rarely encountered today. I have experienced it only once, and that at an Anglican church.
Many composers set the Lamentations of Jeremiah to music. Here is Victoria’s setting of the first reading for Holy Saturday (which is the only one I can find in decent sound).
quia non sumus consumpti:
quia non defecerunt miserationes eius.
Bonum est viro
cum portaverit iugum ab adolescentia sua.
convertere ad Dominum Deum tuum.
It is of the LORD’s mercies
that we are not consumed,
because his compassions fail not.
It is good for a man
that he bear the yoke in his youth.
return unto the Lord thy God.
(Lam. 3: 22, 27)