tenebræ

from The Century Dictionary.

• In the Roman Catholic Church, the matins and lauds of the following day, sung on the afternoon or evening of Wednesday, Thursday, and Friday in Holy Week.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.