The word "tenebrae" is Latin for shadows. The purpose of the Tenebrae service is to recreate the emotional aspects of the passion story, so this is not supposed to be a happy service, because the occasion is not happy. If your expectations of Christian worship is that it should always be happy and exhilarating, you won't appreciate this service until the second time you attend it.
The service was originally designed for Good Friday, but it can be used for Maundy Thursday as well. Both services have long scripture narratives, which for this service are divided into seven, eight or nine parts, each one assigned to a different reader.
The service may include other parts, such as solemn hymns, a sermon, and Communion, but the core of the Tenebrae service works like this: it starts out with the church in candlelight. There are as many candles as there are readings plus a white Christ candle. The readers go up one at a time, read their assigned selections, and extinguish one of the candles, until only the Christ candle remains. Then someone reads the first part of Psalm 22, which Jesus quoted on the cross. The Christ candle is put out, leaving the congregation in near total darkness - and near total devastation. At this point, the service ends. There is no beneduction and the people leave in silence. (The lights are turned up but remain dim so that people can see their way out.)
The purpose of the service is to recreate the betrayal, abandonment, and agony of the events; and it is left unfinished, because the story isn't over until Easter Day.
[Source: Supplement, St. Paul Roman Catholic Cathedral, Saskatoon, SK, Canada; March, 2010]