Tenebrae is Latin for darkness, or shadows, and so this is the name for the service of Matins and Lauds held during Holy Week in the evening.
The gradual extinguishing of candles symbolizes the increasing darkness and sorrow of Jesus' Passion, culminating in the darkness of his death and burial.
Br Paul has been rehearsing the Chant Group for the Gregorian parts of Tenebrae. The choir Caelestis will sing the beautiful polyphonic responses.