The music composed for use by the Catholic church for Holy Week is some of the most poignant and beautiful of all.

Palm Sunday celebrates the entrance of Jesus into Jerusalem. 

On the next day much people that were come to the feast, when they heard that Jesus was coming to Jerusalem, took branches of palm trees, and went forth to meet him and cried, "Hosanna, Blessed is the King of Israel that cometh in the name of the Lord."    St. John 12 and 13

A part of Holy Week is the singing of the Passion. The Passion according to St. Matthew is read on Palm Sunday (those of St. Mark and St. Luke will be read on Tuesday and Wednesday). Also, there is a Palm Sunday procession.

As the procession returns to the door of the church, we have a most beautiful symbolic rite. This return became, naturally, a symbol of our Lord's entry into Jerusalem where he is to suffer. In this people saw a conqueror coming to the place of his triumph. They thought of that entry into Jerusalem as the beginning of his victory, qui immolatus vicerit. So they made a great ceremony of his entrance. One has the picture of a mighty victor thundering at the doors of the city: "Swing back the doors, captains of the guard; swing back, immemorial gates, to let the King enter in triumph" (Ps. 23.7). To welcome Christ, they sent a choir of boys (boys because of the Pueri Hebraeorum) to a gallery or platform above the church doors, to sing the hymn Gloria laus et honor alternately with the procession waiting below. In many mediaeval churches, the Palm Sunday gallery is a feature of the building, over the main doors. In other cases, a temporary platform was set up. The Gloria laus is, without question, one of the most splendid hymns we possess. Unlike most, it is written in a classical metre, in elegiacs. There is a pretty story about the origin of this hymn. It is said that in 828 Theodulph, Bishop of Orleans, was in prison at Angers for having conspired against the Emperor Lewis the Pious, son of Charles the Great (814-840). From his prison, he heard the Palm Sunday procession pass. Then he lifted up his voice and sang out this hymn that he and just composed. The Emperor was in the procession, and was so charmed that he there and then forgave the bishop.

From the book Holy Week by Ronald A. Knox 

Tenebrae

Tenebrae, meaning darkness, describes the singing of the Matins and Lauds offices during the last three days of Holy Week. Normally these offices are sung in the wee hours of the morning in the secluded confines of the convents, but during Holy Week, these offices with their associated sublime singing were moved to an earlier time (to the late afternoon of the day before) and the public invited to participate. The reason that the term darkness is used is to not only to describe the nature of what was taking place (the killing of the being God sent to Earth), but also to describe the gradual extinction of the candles that takes place during these services, leaving the church in total darkness. These services were powerful indeed.

Fifteen unbleached-wax candles are lighted on a triangle called a hearse. The candles are extinguished gradually after each psalm of the office is sung. The final psalm, the Miserere is rendered in total darkness. This was the time that the famous Miserere of Allegri was sung in the Sistine Chapel, and there are powerful and beautiful Misereres composed by other Renaissance and Baroque composers as well. It must have been a powerful event, if one were present in one of the beautiful European cathedrals witnessing one of the beautiful Misereres attributed to Palestrina being sung in total darkness on a Good Friday of long ago.

The Matins and Lauds offices consist of the singing of psalms, prayers and other parts of the service, with a notable part being the singing of the lessons and responsories associated with these offices during the Tenebrae period. The Matins service consist of three parts, called Nocturns, and during each, three  lessons and three responsories are sung.  

After the stripping of the altars, at a suitable hour a signal is given with a clapper, and the clergy assemble for the Maundy. The prelate, or superior, wears a violet stole and cope over amice and alb, and the deacon and subdeacon are vested in white as for the Mass. The superior puts incense into the thurible, assisted by the deacon, who afterwards takes the Gospel book, and kneeling, asks a blessing of the superior. Then attended by two acolytes with lighted candles, he makes the sign of the Cross on the book, which is held by the subdeacon, censes it, and in the usual way sings the Gospel: Ante diem festum. After the Gospel has been sung the subdeacon carries the book to the superior, who kisses it. The superior now removes his cope, and is girded with a towel by the deacon and subdeacon, who accompany him as he proceeds to the washing of the feet.

Those who are to be washed being ranged in order, he kneels before them in turn, and as the subdeacon hold up the right foot of each he washes it, dries it with a towel offered by the deacon, and kisses it. Meanwhile, a number of specified antiphons are sung.

From the book Holy Week by Ronald A. Knox 

This foot-washing ceremony is followed by the Matins service. During the first nocturn, three psalms are sung followed by three lessons and each associated responsory. The second nocturn opens with the signing of three more psalms and three more lesion/responsory pairs, then the third nocturn is sung with three more psalms and three more lesson/responsory pairs. The Office of Lauds follows with the singing of three psalms, the Song of Moses, another psalm, then the Canticle of Zachary. 

All the candles in the triangular candlestick, except the one at the top, have [now] been extinguished during the singing of the Psalms. While the Benedictus is being sung, the six candles on the altar are put out, one at the end of every second verse. All other lights in the church are also extinguished. When the Antiphon Traditor is repeated, the reaming candle is taken from the top of the triangular candlestick and hidden under the Epistle side of the altar. All then keel.

From the book Holy Week by Ronald A. Knox 

Now the wonderful Christus factus is sung, the Pater noster is said silently, then the Psalm Miserere is repeated in a low voice followed by a prayer and the Qui tecum, and a few other items, are said in silence. Then a noise is made, the lighted candle is brought from beneath the altar, and all rise and leave the church in silence.

Good Friday differs greatly from other holy days. For one thing, the mass was not said on Good Friday. Instead, a Mass of the Presanctified is sung, which includes the Passion of St. John sung either using the original Gregorian version, or one of the wonderful setting by Victoria or Lassus with alternating plainsong and polyphonic settings. Following this is the veneration of the cross, filled with ancient and beautiful chants, and perhaps including one of the glorious settings of Pópule meus by Palestrina or Victoria, an perhaps the chorus Crux fidélis, inter omnes by Palestrina. Following this, the beautiful hymn Vexílla Regis is sung.

Holy Saturday has a beautiful Matins and Lauds service, similar to those presented on the two previous days, but using different music and liturgy: each of the three days has its own Matins and Lauds settings. Often the lessons and responsories that were sung were settings by Gallus, Palestrina, Victoria, or Lassus, or by other Renaissance composers for that matter. There are true musical treasures in these compositions. The special service used on Holy Saturday is a Blessing of the Paschal Candle, the chanting of the Prophecies, and the Blessing of the Font.

Holy Week ends with Holy Saturday, then Easter Sunday rings in the new in the celebration of the Resurrection of the Lord Jesus. The music of Easter is joyful and celebrant, in contrast to the sprit of sorrow during the week before. One of the great treasures of Easter is the singing of the sequence Victamae Paschales Laudes. In the protestant church, the spirit of Easter is celebrated in the glorious Cantata #4 of J.S. Bach: Christ Lag in Todesbanden.

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