During the Sacred Triduum , the Matins (now the Office of readings) and Lauds (Morning Prayer) of the Divine Office are often sung in a service known as the Tenebrae service. The service is named Tenebrae because is celebrated in darkness, the name comes from the Latin word for darkness.
During the Matins (now the Office of readings) on Good Friday, one by one, the candles are extinguished in the Church, leaving the congregation in total darkness, and in a silence that is punctuated by the strepitus (a loud clatter intended to evoke the earthquake that was said to happen at the moment of death) meant to evoke the convulsion of nature at the death of Christ. It has also been described as the sound of the tomb door closing.
(adapted from catholic.org)
The Agony in the Garden The name “Tenebrae” has been given because this Office is celebrated in the hours of darkness, formerly in the evening or just after midnight, now the early morning hours. There is an impressive ceremony, peculiar to this Office, which tends to perpetuate its name. There is placed in the sanctuary, near the altar, a large triangular candlestick holding fifteen candles. At the end of each psalm or canticle, one of these fifteen candles is extinguished, but the one which is placed at the top of the triangle is left lighted. During the singing of the Benedictus (the Canticle of Zachary at the end of Lauds), six other candles on the altar are also put out. Then the master of ceremonies takes the lighted candle from the triangle and holds it upon the altar while the choir repeats the antiphon after the canticle, after which she hides it behind the altar during the recitation of the Christus antiphon and final prayer. As soon as this prayer is finished, a noise is made with the seats of the stalls in the choir, which continues until the candle is brought from behind the altar, and shows, by its light, that the Office of Tenebrae is over.