Every single Mass, we hear the words “on the night he was betrayed.”
That night was Holy Thursday, and it is one of the most important nights in all of history.
Here are 10 things you need to know.

1. What happened on the original Holy Thursday?
An amazing amount of stuff! This was one of the most pivotal days in the life of Jesus Christ.
Here are some of the things the gospels record for this day (including events that happened after midnight). Jesus:
  • Sent Peter and John to arrange for them to use the Upper Room to hold the Passover meal.
  • Washed the apostles' feet.
  • Held the first Mass.
  • Instituted the priesthood.
  • Announced that Judas would betray him.
  • Gave the "new commandment" to love one another.
  • Indicated that Peter had a special pastoral role among the apostles.
  • Announced that Peter would deny him.
  • Prayed for the unity of his followers.
  • Held all the discourses recorded across five chapters of John (John 13-18).
  • Sang a hymn.
  • Went to the Mount of Olives.
  • Prayed in the Garden of Gethsemane.
  • Was betrayed by Judas.
  • Stopped the disciples from continuing a violent resistance.
  • Healed the ear of Malchus, the high priest's servant, after Peter cut it off with a sword.
  • Was taken before the high priests Annas and Caiaphas.
  • Was denied by Peter.
  • Was taken to Pilate.
It was a momentous day!
If you'd like to read the gospel accounts themselves, you can use these links:

2. Why is Holy Thursday sometimes called “Maundy Thursday?”
The word “Maundy” is derived from the Latin word mandatum, or “mandate.”
This word is used in the Latin text for John 13:34:
“Mandatum novum do vobis ut diligatis invicem sicut dilexi vos.”
Or, in English:
“A new commandment I give unto you, That ye love one another; as I have loved you.”
Holy Thursday is thus sometimes called Maundy Thursday because it was on this day that Christ gave us the new commandment — the new mandate — to love one another as he loves us.

3. What happens on this day liturgically?
Several things:
  • The bishop celebrates a “Chrism Mass” with his priests (usually).
  • The Mass of the Lord's Supper is held in the evening.
  • At the Mass of the Lord's Supper, the priest (often) performs the washing of feet.
  • The Tabernacle is empty and the Eucharist is put in a place of repose.
  • The altar is stripped.
  • The faithful are invited to spend time in Eucharistic adoration while the Sacrament is in repose.

4. What is the “Chrism Mass?”
According to the main document governing the celebrations connected with Easter, Paschales Solemnitatis:
35. The Chrism Mass which the bishop concelebrates with his presbyterium and at which the holy chrism is consecrated and the oils blessed, manifests the communion of the priests with their bishop in the same priesthood and ministry of Christ.
The priests who concelebrate with the bishop should come to this Mass from different parts of the diocese, thus showing in the consecration of the chrism to be his witnesses and cooperators, just as in their daily ministry they are his helpers and counselors.
The faithful are also to be encouraged to participate in this Mass, and to receive the sacrament of the Eucharist.
Traditionally the Chrism Mass is celebrated on the Thursday of Holy Week. If, however, it should prove to be difficult for the clergy and people to gather with the bishop, this rite can be transferred to another day, but one always close to Easter.
The chrism and the oil of catechumens is to be used in the celebration of the sacraments of initiation on Easter night.

5. Why is the Mass of the Lord's Supper significant?
According to Paschales Solemnitatis:
45. Careful attention should be given to the mysteries which are commemorated in this Mass: the institution of the Eucharist, the institution of the priesthood, and Christ's command of brotherly love; the homily should explain these points.

6. Is the Eucharist in the Tabernacle during this Mass?
No. According to Paschales Solemnitatis:
48. The Tabernacle should be completely empty before the celebration.
Hosts for the Communion of the faithful should be consecrated during that celebration.
A sufficient amount of bread should be consecrated to provide also for Communion on the following day.

7. What does the rite of foot washing signify, and is it to be done for men only?
According to Paschales Solemnitatis:
51. The washing of the feet of chosen men which, according to tradition, is performed on this day, represents the service and charity of Christ, who came "not to be served, but to serve. This tradition should be maintained, and its proper significance explained.
Although some have interpreted the rite as reflecting the institution of the institution of the priesthood or being unique to the apostles, this interpretation is not found in the Church's official documents, such as Paschales Solemnitatis, which interpret it as a sign of service and charity.
The rite is optional. It does not have to be performed.
Although until 2016 the Church’s official texts used language that indicated only men (Latin, viri) could have their feet washed on Holy Thursday, the Holy See had permitted individual bishops to wash the feet of females and younger males (vir means "man," not "male") for some time.
Pope Francis himself had been doing so, and in 2016 he had the Congregation for Divine Worship revise the law to bring it into alignment with contemporary practice.
You can read the decree that did so here.

8. What happens at the end of the Mass of the Lord's Supper?
According to Paschales Solemnitatis:
54. After the post-Communion prayer, the procession forms, with the crossbar at its head. The Blessed Sacrament, accompanied by lighted candles and incense, is carried through the church to the place of reservation, to the singing of the hymn "Pange lingua" or some other eucharistic song.
This rite of transfer of the Blessed Sacrament may not be carried out if the Liturgy of the Lord's Passion will not be celebrated in that same church on the following day.
55. The Blessed Sacrament should be reserved in a closed tabernacle or pyx. Under no circumstances may it be exposed in a monstrance.
The place where the tabernacle or pyx is situated must not be made to resemble a tomb, and the expression "tomb" is to be avoided.
The chapel of repose is not prepared so as to represent the "Lord's burial" but for the custody of the eucharistic bread that will be distributed in Communion on Good Friday.

9. Is there to be Eucharistic adoration at this time?
According to Paschales Solemnitatis:
56. After the Mass of the Lord's Supper the faithful should be encouraged to spend a suitable period of time during the night in the church in adoration before the Blessed Sacrament which has been solemnly reserved.
Where appropriate, this prolonged eucharistic adoration may be accompanied by the reading of some part of the Gospel of St. John (chs. 13-17).
From midnight onwards, however, the adoration should be made without external solemnity, because the day of the Lord's passion has begun.

10. What happens to the decoration of the Church at this time?
According to Paschales Solemnitatis:
57. After Mass the altar should be stripped.
It is fitting that any crosses in the church be covered with a red or purple veil, unless they have already been veiled on the Saturday before the Fifth Sunday of Lent.
Lamps should not be lit before the images of saints.