The Office of Tenebrae
The public singing of part of the Divine Office, on Wednesday, Thursday and Friday evenings of Holy Week, anticipating Matins and Lauds of Holy Thursday, Good Friday, and Holy Saturday. A custom that went back centuries, it acquired the name because of the mourning ritual surrounding the ceremony, which included a triangular stand with fifteen candles. These were put out one by one until, after the last candle was extinguished, a prayer was said in darkness, one candle was lighted, and the assembly dispersed in silence. — Fr John Hardon, Modern Catholic Dictionary
The Matins and Lauds of the Divine Office sung during the Triduum (Holy Thursday, Good Friday and Holy Saturday) are known as the Tenebrae services (“tenebrae” meaning “darkness or shadows”), which is basically a funeral service for Our Lord.
During the Matins 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 clang intended to evoke the earthquake that occurred at the moment of Our Saviour’s 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.