The Easter Triduum (sometimes called the Paschal Triduum) begins on Holy Thursday with the evening Mass of the Lord’s Supper, is continued through Good Friday with the celebration of the Passion of the Lord and Holy Saturday, reaches its summit in the Easter Vigil, and concludes with Vespers (evening prayer) of Easter Sunday.
Holy Thursday is the day on which Christ celebrated the Last Supper with His disciples. This is the day we celebrate the institution of the Eucharist as the true body and blood of Jesus Christ and the institution of the sacrament of the priesthood.
During the Last Supper, Christ blessed the bread and wine with the same words that Catholic and Orthodox priests use today to consecrate the Body and Blood of Christ during the Mass. In telling His disciples to “Do this in remembrance of Me,” He instituted the Mass and made them the first priests. During the Last Supper, Jesus offers himself as the Passover sacrifice, the sacrificial lamb, and teaches that every ordained priest is to follow the same sacrifice in the exact same way.
The washing of the feet represents the service and charity of Christ, who came “not to be served, but to serve.”
It was only a few hours after the Last Supper that Judas would betray Christ in the Garden of Gethsemane, setting the stage for Christ’s Crucifixion on Good Friday.
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.
At the conclusion of the Mass, the faithful are invited to continue Adoration of the Blessed Sacrament throughout the night, just as the disciples were invited to stay up with the Lord during His agony in the garden before His betrayal by Judas.
After Holy Thursday, no Mass will be celebrated again in the Church until the Easter Vigil celebrates and proclaims the Resurrection of the Lord Jesus Christ.