Tenebrae Prayer Service

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