from Wikipedia:

The traditional Roman Catholic Tenebrae is a combination of Matins (composed of 3 nocturns each day) and Lauds, the first two hours of the Divine Office. The readings of each day's first nocturn are taken from the Book of Lamentations. Each day, the office of Tenebrae contains 14 psalms, 9 readings, and one canticle, the Benedictus (Song of Zechariah). Lighting is gradually reduced throughout the service. Initially 15 candles are lit and are placed on a special stand known as a hearse, which are extinguished one by one after each psalm. The last candle is hidden beneath the altar, ending the service in total darkness. In some places the use of a strepitus (Latin for "great noise") is included as part of the service. The great noise is usually generated by slamming a book closed, banging a hymnal or breviary against the pew, or stomping on the floor, symbolizing the earthquake that followed Christ's death. This custom seems to have originated as a simple signal to depart in silence.^[1] Following the great noise a single candle, which had been hidden from view is returned to the top of the hearse. It is felt that the single candle signifies the return of Christ to the world with the Resurrection.The lessons from the Lamentations of Jeremiah the Prophet have been set to music by many composers, of whom the most famous are Palestrina, Tallis, Lassus, Marc-Antoine Charpentier, Franois Couperin, Ernst Krenek (Lamentatio Jeremiae prophetae, op. 93) and Stravinsky (Threni). In addition, the responses have been set by Lassus and Gesualdo.

Over forty years ago I had the pleasure of being part of the chorus of St. Joseph's Seminary, Dunwoodie, New York which sang the setting of the "Lamentations of Jeremiah" by Franois Couperin (1668-1733). Couperin's "Leons de Tnbres" is available on a fine recording with the counter-tenor Ren Jacobs on the harmonia mundi label.But even more memorable is the recollection of the "Tenebrae" service prayed and chanted the mornings of Holy Thursday, Good Friday, and Holy Saturday. In many seminaries, and some parishes, some form of "Tenebrae" is prayed. Participation in them makes for wonderful preparation for the major liturgies of these holy days.For Holy Thursday, the patristic reading at Tenebrae is from a homily of Melito of Sardis. It says in part:

Christ is the One who brought us out of slavery into freedom, out of darkness into light, out of death into life, out of tyranny into an eternal kingdom. He made us a new priesthood, a people chosen to be his own forever.Christ is the Passover that is our salvation.

A blessed Triduum to all.Addendum:Interesting reflections at Busted Halo from a young Notre Dame graduate who first experienced "Tenebrae" at the university:

For Catholics, good liturgy has always aimed to engage the body and its senses. We kneel, sit and stand throughout the Mass, and cross our brows, mouths and hearts before the proclamation of the Gospel. We taste the body and blood of Christ and inhale the perfume of incense. On special occasions, we rub our palm branches between our fingers, or smudge the ashes on our forehead. We listen, and we watch.Our tactile impulses certainly enhance the appeal of hammering away on the nearest pew at the conclusion of Tenebrae. But the services chief sensory aim is to deprive us of our vision, in a startling way that feels fresh, and forces us to shed our sometimes-rote relationship to the Mass. By the end of the service, all that remains is a church full of shadowsthe sheer absence we experience during Lent is made real and sacramental.