The memorial of the Lord's Passion and Resurrection.
The Holy Sacrifice, because it makes present the one sacrifice of Christ the Savior and includes the Church's offering. The terms holy sacrifice of the Mass, "sacrifice of praise," spiritual sacrifice, pure and holy sacrifice are also used, since it completes and surpasses all the sacrifices of the Old Covenant.
The Holy and Divine Liturgy, because the Church's whole liturgy finds its center and most intense expression in the celebration of this sacrament; in the same sense we also call its celebration the Sacred Mysteries. We speak of the Most Blessed Sacrament because it is the Sacrament of sacraments. The Eucharistic species reserved in the tabernacle are designated by this same name.
Source: Catechism of the Catholic Church (Vatican.va)