Well, from the "horse's mouth":
1366 The Eucharist is thus a sacrifice because it re-presents (makes present) the sacrifice of the cross, because it is its memorial and because it applies its fruit:
[Christ], our Lord and God, was once and for all to offer himself to God the Father by his death on the altar of the cross, to accomplish there an everlasting redemption. But because his priesthood was not to end with his death, at the Last Supper "on the night when he was betrayed," [he wanted] to leave to his beloved spouse the Church a visible sacrifice (as the nature of man demands) by which the bloody sacrifice which he was to accomplish once for all on the cross would be re-presented, its memory perpetuated until the end of the world, and its salutary power be applied to the forgiveness of the sins we daily commit.
1367 The sacrifice of Christ and the sacrifice of the Eucharist are one single sacrifice: "The victim is one and the same: the same now offers through the ministry of priests, who then offered himself on the cross; only the manner of offering is different." "And since in this divine sacrifice which is celebrated in the Mass, the same Christ who offered himself once in a bloody manner on the altar of the cross is contained and is offered in an unbloody manner. . . this sacrifice is truly propitiatory."
1370 To the offering of Christ are united not only the members still here on earth, but also those already in the glory of heaven. In communion with and commemorating the Blessed Virgin Mary and all the saints, the Church offers the Eucharistic sacrifice. In the Eucharist the Church is as it were at the foot of the cross with Mary, united with the offering and intercession of Christ.
That's so ridiculous. Humans didn't offer Christ as a sacrifice to God.
The only human contribution and participation to the death of Christ is putting Him on the cross and essentially pounding the nails in His hands.
Jesus was not a sacrifice offered by men to God for their sins because the lambs weren't good enough. It wasn't man's idea nor man's doing.
Thanks for the reply, boatbums.
“1367 The sacrifice of Christ and the sacrifice of the Eucharist are one single sacrifice:...”
So the sacrifice of the Eucharist came first? It “made present” Christ’s sacrifice before it happened. Really, annalex, really?
Boatbums: [quotes canons 1366, 1367 without comment] 1366 The Eucharist is thus a sacrifice because it re-presents (makes present) the sacrifice of the cross, because it is its memorial and because it applies its fruit:
[Christ], our Lord and God, was once and for all to offer himself to God the Father by his death on the altar of the cross, to accomplish there an everlasting redemption. But because his priesthood was not to end with his death, at the Last Supper "on the night when he was betrayed," [he wanted] to leave to his beloved spouse the Church a visible sacrifice (as the nature of man demands) by which the bloody sacrifice which he was to accomplish once for all on the cross would be re-presented, its memory perpetuated until the end of the world, and its salutary power be applied to the forgiveness of the sins we daily commit.
1367 The sacrifice of Christ and the sacrifice of the Eucharist are one single sacrifice: "The victim is one and the same: the same now offers through the ministry of priests, who then offered himself on the cross; only the manner of offering is different." "And since in this divine sacrifice which is celebrated in the Mass, the same Christ who offered himself once in a bloody manner on the altar of the cross is contained and is offered in an unbloody manner. . . this sacrifice is truly propitiatory."
So, as you now plainly see, the sacrifice of the Mass is not a repetition, and not a performance by the priest. (If you are willing to say that Christ "performed" his sacrifice on the Cross, then I guess it is "performance" by Christ).