Part II-4. Celebration of the Eucharist in the Life and Ministry of the Bishop and Priest
42. The celebration of the eucharist expresses in a special way the public and social nature of the liturgical celebrations of the Church, "which is the sacrament of unity, namely, the holy people united and ordered under their bishops." 
Hence "marked with the fullness of the sacrament of orders, a bishop is the steward of the grace of the supreme priesthood, especially in the eucharist, which he offers or causes to be offered . . . Every lawful celebration of the eucharist is regulated by the bishop, to whom is committed the office of offering the worship of Christian religion to the divine majesty and of administering it in accordance with the Lord's commandments and the Church's laws, as further defined by his particular judgment for his diocese."  The celebration of the eucharist at which the bishop presides, surrounded by his college of priests and ministers, with the whole people of God actively taking part, is the preeminent manifestation of the hierarchically constituted Church. 
Appropriateness of the Participation of Priests in the Eucharist, Exercising Their Proper Office
43. In the celebration of the eucharist, priests also are deputed, by reason of a special sacrament, namely, orders, to fulfill the office proper to them. For they too "as ministers of the sacred, especially in the sacrifice of the Mass, . . . represent the person of Christ in a particular way."  Because of the sign value, it is therefore right that they take part in the eucharist by exercising the order proper to them,  that is, by celebrating or concelebrating the Mass and not simply by receiving communion like the laity.
Daily Celebration of Mass
44. "In the mystery of the eucharistic sacrifice, the fulfillment of the priest's chief office, the work of redemption is continually actual. Hence, daily celebration is urged upon priests; it remains the act of Christ and the Church even when the faithful cannot attend";  in it the priest always acts for the salvation of the people.
Faithful Observance of the Laws of the Church in Celebrating Mass
45. Especially in the celebration of the eucharist, only the supreme authority of the Church and, according to the norm of law, the bishops and the conferences of bishops, no one else, not even a priest, may, on his own initiative add, leave out, or change anything in the liturgy.  Therefore, priests should be intent on presiding over the celebration of the eucharist in such a way that the faithful know that they are participating not in a rite decided on by private authority,  but in the public worship of the Church, the direction of which has been entrusted by Christ to the apostles and their successors.
Priority of Pastoral Effectiveness in the Choice of the Different Forms of Celebration
46. "When the liturgy is celebrated, something more is required than the mere observance of the laws governing valid and lawful celebration; it is also their duty to ensure that the faithful take part fully aware of what they are doing, actively engaged in the rite, and enriched by its effect."  Hence, from among the forms of celebration permitted by law priests should take care to choose those that in each situation seem best suited to the needs or well-being of the faithful and to their taking part actively.
47. Concelebration of the eucharist aptly expresses the unity of the sacrifice and the priesthood; whenever the faithful take an active part, the unity of the people of God stands out in a special way,  particularly if the bishop presides. 
Concelebration also symbolizes and strengthens the fraternal bond between priests, because "by virtue of the ordination to the priesthood that they share all are linked together in a close bond of brotherhood." 
Unless the needs of the faithful (which always must be regarded with a deep pastoral concern) rule it out, then, and without prejudice to the option of every priest to celebrate Mass individually, this excellent way for priests to celebrate Mass is preferable in the case of communities of priests, their periodic meetings, or in other similar circumstances. Those who live in community or serve the same church should gladly welcome visiting priests to concelebrate with them.
The authorized superiors should therefore facilitate and encourage concelebration whenever pastoral needs or another reasonable cause does not demand otherwise.
The faculty to concelebrate also applies to the principal Masses in churches and public and semipublic oratories of seminaries, colleges, and ecclesiastical institutions, as well as in those of religious orders and societies of common life without vows. Where there are a great many priests, the authorized superior can allow several concelebrations to take place on the same day, but at different times or in different places of worship.
Baking of the Bread for Concelebration
48. If a large host is baked for concelebration, as permitted in the Rite of Concelebration no. 17, care must be taken that, in keeping with traditional usage, it is of a form and appearance worthy of the eucharistic mystery.