TENEBRAE
Etymology
Proper noun
Tenebrae
(Christianity) A Christian religious service celebrated by the Western Church on the evening before or early morning of Maundy Thursday, Good Friday, and Holy Saturday, involving the gradual extinguishing of candles while a series of readings and psalms are chanted or recited.
Source: Wiktionary
Ten"e*bræ, n. Etym: [L., pl., darkness.] (R. C. Ch.)
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition