Jesus offers himself in sacrifice to the Father, which he unites with the bread and wine at the Last Supper. When he tells his Apostles, "Do this in memory of me," he institutes the Eucharistic Sacrifice, which is what the Mass is. We are not offering a new sacrifice day after day, like they did in the Old Covenant, but we are participating anew in the one sacrifice of Jesus on the Cross, united with the sacrifice of the Last Supper.