The word tenebrae means “darkness” and refers specifically to a service of worship in which the progressive extinguishing of candles represent the snuffing out of the life of Christ.
The word tenebrae appears after the twelfth century and is used to refer to Matins and Lauds (prayers at dawn and sunset) on the last three days of Holy Week. These services were sung on the night before the day to which they refer, beginning on Wednesday night before Maundy Thursday. Originally the Matins and Lauds for the three great days were simply the ordinary office sung in monastic communities of the medieval era.
The Matins and Lauds for the last three days of Holy Week consisted of a series of the events of salvation that took place from the Last Supper to the entombment. On Thursday the betrayal is emphasized; on Friday the judgment, crucifixion, and death of Christ; and on Saturday the burial and expected resurrection.
The services featured a triangular candlestick holding fifteen candles. Nine for the Matins reading and five for the Lauds, with one remaining. Although the one remaining may have originally been for the purpose of light for the departing worshipers, it gradually came to represent the Resurrection and the enduring light of Jesus.
The service begins with all fifteen candles lighted. Then the minister, readers, and choir or singer process into the sanctuary in silence. The first antiphon is read or sung, followed by the reading or singing of the Psalm. Then a candle is snuffed out. This order of service progresses throughout until all the candles except the middle (or top) one has been snuffed out (in addition to the Psalms, a candle is snuffed out after the reading of the Song of Hezekiah).
By the time Canticle 16: Benedictum Dominus Deus Israel is sung, only one candle is burning. During the Benedictus the only lighted candle may be taken and hidden in the sanctuary to represent the death and burial of Jesus. After Psalm 51 has been said, a noise may be made representing the opening of the grave. The remaining candle representing the Resurrection may be returned to the candlestick to provide light for the people as they leave the sanctuary in silence.