With this Mass, celebrated in the evening of the Thursday in Holy Week, the Church begins the sacred Easter Triduum and devotes herself to the remembrance of the Last Supper. At the super on the night he was betrayed, the Lord Jesus, loving those who were his own in the world even to the end, offered his Body and Blood to the Father under the appearance of bread and wine, gave them to the apostles to eat and drink, then enjoined the apostles and their successors in the priesthood to offer them in turn.
This Mass is, first of all, the memorial of the institution of the eucharist, that is, of the Memorial of the Lord’s Passover, by which under sacramental signs he perpetuated among us the sacrifice of the New Law. The Mass of the Lord’s Supper is also the memorial of the institution of the priesthood, by which Christ’s mission and sacrifice are perpetuated in the world. In addition, this Mass is the memorial of that love by which the Lord loved us even to death.
Source: Ceremonial of Bishops, no. 297.