Tenebrae literally means darkness or shadows. Beginning in the Medieval period Tenebrae was the name given to the forms of prayers, songs, and grieving that characterized the last three days of Holy Week. These services were often observed at monasteries.
The name most probably came from the fact that these forms of prayer and song included the extinguishing of candles one by one in the church during the service. In the traditional services there were fifteen candles, fourteen dark and one white. Medieval services included fourteen readings from the Psalms.
Since the 1950s Tenebrae more commonly refers to a service held on Good Friday that includes the story of Jesus’ passion, mournful song, and the extinguishing of candles. The Tenebrae service has a somber tone set by growing darkness in the sanctuary, the accounts of Christ’s suffering, music, and silence.