A short description of Tenebrae from our friars of Western Dominican Province:
From the Latin word for “darkness,” Tenebrae is the term given to the liturgical office of Holy Thursday, Good Friday, and Holy Saturday as they were observed prior to the reform of Holy Week by Pope Pius XII in 1955. Dominicans have continued to pray Tenebrae each year as a particular tradition of our Order.
The practice of Tenebrae has roots as early as the 7th century, when those celebrating the Office would do so in almost complete darkness; the only light coming from a large candelabra called a hearse.
In the contemporary rite, the Office contains five psalms and one canticle. After each psalm or canticle, a set of candles is extinguished, representing the fleeing of the Apostles, until there is only one left, the so-called Christ candle.