“Tenebrae” is the name given to the celebration, with special ceremonies, of Matins and Lauds, the first two hours of the Divine Office, of the last three days of Holy Week. The traditions regarding this service go back at least to the ninth century. Originally celebrated after midnight, by the late Middle Ages their celebration was anticipated on the afternoon or evening of the preceding day.
The celebration of Matins and Lauds of these days in the form referred to as Tenebrae in churches with a sufficient number of clergy was universal in the Roman Rite until the reform of the Holy Week ceremonies by Pope Pius XII in 1955. At that time, the Easter Vigil was restored as a night office, moving that Easter liturgy from Holy Saturday morning to the following night; the principal liturgies of Holy Thursday and Good Friday were likewise moved from morning to afternoon or evening, and thus Matins and Lauds were no longer allowed to be anticipated on the preceding evening, except for the Matins and Lauds of Holy Thursday in the case of cathedral churches in which the Chrism Mass was held on Holy Thursday morning.