The name of Tenebrae has been given to the Matins (early morning, when still dark) and Lauds (first light, dawn, 6am or so) of the last three days of Holy Week (Holy Thursday, Good Friday, Holy Saturday).

There is placed in the sanctuary, near the altar, a large triangular candlestick, holding fifteen candles. These candles, and the six that are on the altar, are of yellow was, as in the Office for the dead. At the end of each psalm or canticle, one of these fifteen candles is extinguished; but the one which is placed at the top of the triangle is left lighted. During the singing of the Benedictus, at Lauds; the six candles on the altar are also put out. Then the master of ceremonies takes the lighted candle from the triangle, and holds it upon the altar, on the epistle side, while the choir repeats the antiphon after the canticle: after which he hides it behind the altar during the recitation of the Miserere and the prayer which follows the psalm. As soon as this prayer is finished, a noise is made with the seats of the stalls in the choir, which continues until the candle is brought from behind the altar, and shows, by its light, that the Office of Tenebrae is over.
 
What is the meaning of this prayer? Let us now study the meaning of these ceremonies. The glory of the Son of God was obscured, and, so to say, eclipsed, by the ignominies He endured during His Passion. He, the Light of the world, powerful in word and work, who, but a few days ago, was proclaimed King by the citizens of Jerusalem, is now robbed of all His honours; He is, says Isaias, the Man of sorrows, a leper (Isaiah 53:3-4); He is, says the royal prophet, a worm of the earth, and not man (Psalm 22:7); he is, as He says of Himself, an object of shame even to His own disciples, for they are all scandalized in Him, (Mark 14:27) and abandon Him; yea, even Peter protests that he never knew Him. This desertion on the part of His apostles and disciples is expressed by the candles being extinguished, one after the other, not only on the triangle, but on the altar itself. But Jesus, our Light, though despised and hidden, is not extinguished. This is signified by the candle which is momentarily placed on the altar; it figures our Redeemer suffering and dying on Calvary. In order to express His burial, the candle is hidden behind the altar; its light disappears. A confused noise is heard in the house of God, where all is now darkness. This noise and gloom express the convulsions of nature, when Jesus expired on the cross: the earth shook, the rock were split, the dead came forth from their tombs. But the candle suddenly reappears; its light is as fair as ever; the noise is hushed, and homage is paid to the Conqueror of death.

To pray Tenebrae the traditional way it should be prayed all three days, Holy Thursday, Good Friday, and Holy Saturday.

If Tenebrae is prayed in the evenings, the same structure is kept, but the Canticle of Zachariah is replaced with with the Canticle of Mary, and the altar candles are extinguished at that time.
 
A very short version of Tenebrae, done either in the morning or evening could be done by simply chanting the antiphons only and extinguishing the candles after the antiphons. Lessons would still be read and the Canticles would be chanted.

Since the theme of this packet is Holy Thursday, only Tenebrae for Holy Thursday is included.

This video edited to show portions of Tenebrae.

Source: Abbot Gueranger, O.S.B.; The Liturgical Year; Vol. 6; Passiontide & Holy Week