Holy Thursday is the day that Christ celebrated the Last Supper with His disciples, four days after His triumphal entry into Jerusalem on Palm Sunday. Only hours after the Last Supper, Judas would betray Christ in the Garden of Gethsemane, setting the stage for Christ's Crucifixion on Good Friday.
This feast, however, is more than just the lead-in to Good Friday; it is, in fact, the oldest of the celebrations of Holy Week. And with good reason: Holy Thursday is the day that Catholics commemorate the institution of three pillars of the Catholic Faith: the Sacrament of Holy Communion, the priesthood, and the Mass. During the Last Supper, Christ blessed the bread and wine with the very words that Catholic and Orthodox priests use today to consecrate the Body and Blood of Christ during the Mass and the Divine Liturgy. In telling His disciples to "Do this in remembrance of Me," He instituted the Mass and made them the first priests.
Near the end of the Last Supper, after Judas had departed, Christ said to His disciples, "A new commandment I give unto you: That you love one another, as I have loved you, that you also love one another." The Latin word for "commandment," mandatum became the source for another name for Holy Thursday: Maundy Thursday.
On Holy Thursday, the priests of each diocese gather with their bishop to consecrate holy oils, which are used throughout the year for the sacraments of Baptism, Confirmation, Holy Orders, and the Anointing of the Sick. This ancient practice, which goes back to the fifth century, is known as the Chrism Mass ("chrism" is a mixture of oil and balsam used for the holy oils) and stresses the role of the bishop as a successor to the apostles.
Except in very rare circumstances, there is only one Mass other than the Chrism Mass celebrated on Holy Thursday in each church: the Mass of the Lord's Supper, which is celebrated after sundown. It commemorates the institution of the Sacrament of Holy Communion, and it ends with the removal of the Body of Christ from the tabernacle in the main body of the church. The Eucharist is carried in procession to another place where it is kept overnight, to be distributed during the commemoration of the Lord's Passion on Good Friday. After the procession, the altar is stripped bare, and all bells in the church are silent until the Gloria at the Easter Vigil on Holy Saturday.