In the Offertory, where the Church offers her sacrifice of thanks and praise, in the bread and wine (and perhaps other gifts), the priest, in the Ordinary form of the Latin rite of the Roman Catholic Church, says (amongst other things) to the congregation (emphasis added): “Pray, brethren (brothers and sisters), that my sacrifice and yours may be acceptable to God, the almighty Father.” This is the wording of the current english translation of the Roman Rite, which was implemented on the 1st Sunday of Advent 2011. Before, the english text said (emphasis added): “Pray, brethren, that our sacrifice may be acceptable to God, the almighty Father.”
The question here, is: What is meant by ‘my sacrifice and yours,’ and how does this differ from ‘our sacrifice,’ which is a rather loose translation?
The latin text behind both versions is meum ac vestrum sacrificium. Translated directly, it says ‘my and yours sacrifice’ (or ‘my sacrifice and yours,’ which flows better). ‘Our sacrifice’ may be a passable translation, but it is not very good. To understand this, we need to ask why the Latin original uses meum ac vestrum sacrificium (‘my and yours sacrifice’/‘my sacrifice and yours’), and not merely (something like) sacrificium nostrum (‘our sacrifice’).
The reason for this, is that meum ac vestrum sacrificium points to an important difference between the priest and the laity at Mass or, rather, the difference between the presiding priest(s) and everyone else at Mass, including other priests. In the offering of the Sacrifice of Mass, the priest offers the sacrifice ‘in the person of Christ’ (in persona Christi). But there is only one offering. The sacrifice of Christ is the sacrifice of the people. Just as the High Priest of the Old Covenant, having offered for himself, offered the sacrifice of the people on their behalf, Christ offers the sacrifice of the Church on her behalf. The sacrifice which is offered by the people under Mass is the same offering that the priest offer – Christ himself, to the Father. The difference is that the people represent themselves, while the priest represents Christ.
As we see in my master’s thesis, from my analysis of Joseph Ratzinger’s view concerning the sacrifice of Christ and the Eucharist, Ratzinger sees the roles of Christ, the Church and the priest in the Eucharistic celebration as part of one, integral whole. There is but one sacrifice; the thank offering of Christ, and this he offers in heaven, while his priests offer this in persona Christi on earth. Yet his Church is also offering this sacrifice by participating in Christ. The reason for this is that the sacrifice of Christ is the sacrifice of the Church, the sacrifice of humanity, offered up by Christ, who is our representative. This is not a novel idea in Catholic theology. We find it, for example, in the 1979 Elucidation of the statement on ministry and ordination in the documents from the Anglican-Catholic dialogue (ARCIC):
[The] ordained ministry is called priestly principally because it has a particular sacramental relationship with Christ as High Priest. At the eucharist Christ’s people do what he commanded in memory of himself and Christ unites them. sacramentally with himself in his self-offering. But in this action it is only the ordained minister who presides at the eucharist, in which, in the name of Christ and on behalf of his Church, he recites the narrative of the institution of the Last Supper, and invokes the Holy Spirit upon the gifts.
It’s also found in Mediator Dei, an encyclical of Pope Pius XII from 1947:
Now it is clear that the faithful offer the sacrifice by the hands of the priest from the fact that the minister at the altar, in offering a sacrifice in the name of all His members, represents Christ, the Head of the Mystical Body. Hence the whole Church can rightly be said to offer up the victim through Christ. But the conclusion that the people offer the sacrifice with the priest himself is not based on the fact that, being members of the Church no less than the priest himself, they perform a visible liturgical rite; for this is the privilege only of the minister who has been divinely appointed to this office: rather it is based on the fact that the people unite their hearts in praise, impetration, expiation and thanksgiving with prayers or intention of the priest, even of the High Priest himself, so that in the one and same offering of the victim and according to a visible sacerdotal rite, they may be presented to God the Father. It is obviously necessary that the external sacrificial rite should, of its very nature, signify the internal worship of the heart. Now the sacrifice of the New Law signifies that supreme worship by which the principal Offerer himself, who is Christ, and, in union with Him and through Him, all the members of the Mystical Body pay God the honor and reverence that are due to Him.
I think that although ‘our sacrifice’ may be a passable translation of meum ac vestrum sacrificium, it ought to be translated ‘my sacrifice and yours,’ since this underlines the different roles of the priest and the laity. But it needs to be stressed that this is not two (or more) sacrifices. It is the same sacrifice, which is clear in that in both the english and latin version, the word ‘sacrifice’ (sacrificium) is singular.
Lastly, some might say that this is not the offering of Christ, since it is the Offertory and no consecration has been made. Yes, no consecration has been made at this point in Mass, yet the Offertory is part of the things Christ did (the offering of bread and wine), the things he said should be done in remembrance of him. It is a offering of thanks an praise through Christ, and is anticipatory of the consecration.
 This can be found on the web page of the United States Conference of Catholic Bishops (USCCB) ( here and here)
, or in various books forms, such as The Order of Mass in Latin and English (New English Translation. London: CTS 2011).
 The Catechism of the Catholic Church 1548-1551; Lumen Gentium 10, 28; Sacrosanctum Concilium 33; Christus Dominus 11; Presbyterorum Ordinis 2, 6; St. Thomas Aquinas, ST 3a, 22.4. Also see pope Benedict’s Munus docendi and Jospeh Ratzinger, The Spirit of the Liturgy (San Francisco, CA: Ignatius Press 2000): 171–177. This is also part of Lutheran belief, as held by Wolfhart Pannenberg, in Systematic Theology, vol. 3 (Edinburgh: T&T Clark 1998): 389 (cf. Baptism, Eucharist and Ministry (the Lima report) 2:14, with commentary). A quotation can be found on p.33 of my master’s thesis. Also see this document from the 11th Plenary meeting of the Lutheran-Orthodox Joint Commission, held in Oslo, Oct. 3-10, 2002, and Rev. Rodney L. Eberhardt, “The Pastor as In Persona Christi” (lecture at the Society of the Holy Trinity General Retreat, Sept. 29, 2009).
 Christ, of course, did not offer for himself. See Matt. 4:1-11; Heb. 2:17, 4:14-15; 5:1-3; 7:27.
 Ratzinger, The Feast Of Faith: Approaches to a Theology of the Liturgy (Translated by Graham Harrison. San Francisco, CA: Ignatius Press 1986): 50-60; Jesus of Nazareth, vol. 2 (English translation provided by the Vatican Secretariat of State. San Francisco, CA: Ignatius Press 2011): 1-2.76-90.115-138.223-240; Pope Benedict, Deus Caritas Est 12-13; The Spirit of the Liturgy, pp. 37-50.53-61.171–177; Munus docendi, cf. Rom. 12:1; 1Pet 2:5; Heb. 8:1-3; 9:11-12.
 Cf. Consecrated Women? A Contribution to the Women Bishops Debate (ed., Jonathan Baker. Norwich: Canterbury Press 2004): 56-57, cf. 48-58.
 Mediator Dei 93. Encyclical of Pope Pius XII on the sacred liturgy, 1947. Also see Cyprian Vagaggini, Theological Dimensions of the Liturgy (Translated by Leonard J. Doyle and W.A. Jurgens from the fourth italian edition. Collegeville, MN: Liturgical Press 1976): 153-156.