The Collaborative International Dictionary of English v.0.48:

Tenebrae \Ten"e*brae\, n. [L., pl., darkness.] (R. C. Ch.)
   The matins and lauds for the last three days of Holy Week,
   commemorating the sufferings and death of Christ, -- usually
   sung on the afternoon or evening of Wednesday, Thursday, and
   Friday, instead of on the following days.
   [1913 Webster]