Catechism of the Catholic Church
“In the sense of Sacred Scripture the memorial is not merely the recollection of past events but the proclamation of the mighty works wrought by God for men. In the liturgical celebration of these events, they become in a certain way present and real. This is how Israel understands its liberation from Egypt: every time Passover is celebrated, the Exodus events are made present to the memory of believers so that they may conform their lives to them. In the New Testament, the memorial takes on new meaning. When the Church celebrates the Eucharist, she commemorates Christ’s Passover, and it is made present the sacrifice Christ offered once for all on the cross remains ever present. ‘As often as the sacrifice of the Cross by which ‘Christ our Pasch has been sacrificed’ is celebrated on the altar, the work of our redemption is carried out.'”
Source: Catechism of the Catholic Church #1363-4