Prayer to St. Michael & the Mass
One of the purposes of the liturgical renewal of the Second Vatican Council was to instill in the faithful a proper sense of the liturgy. As baptized Christians the laity possess the common priesthood of Christ, by which as members of Christ they share in Christ's worship. The ordained or ministerial priest takes his place in that worship as the sacramental sign of Christ the Head, so that the whole Christ, Head and members, offer fitting worship to God the Father, through, with and in Christ, and in the Holy Spirit.
Among the ways this renewal was effected was to restore active participation to the laity, by emphasizing the liturgical prayers and actions that are proper to them (and which were often done by the servers or choir in their name), and, to remove elements from the Mass which were devotional, rather than liturgical (part of the Mass proper). Among the non-liturgical elements that were removed were the Prayers at the end of the Mass, including the Prayer to St. Michael. In this way the common liturgical celebration ends with the dismissal proper to the Mass.
In 1994 Pope St. John Paul II requested the faithful to take up again the praying of the Prayer to St. Michael in the battle of our times "against the forces of darkness and against the spirit of this world."
May prayer strengthen us for the spiritual battle we are told about in the Letter to the Ephesians, "Draw strength from the Lord and from his mighty power" (Eph 6 10). The Book of Revelation refers to this same battle recalling before our eyes the image of St. Michael the Archangel (Rev. 12:7). Pope Leo XIII certainly had a very vivid recollection of this scene when, at the end of the last century, he introduced a special prayer to St Michael throughout the Church. "St Michael the Archangel defend us in battle, be our safeguard against the wickedness and snares of the devil." Although today this prayer is no longer recited at the end of Mass, I ask everyone not to forget it, and to recite it to obtain help in the battle against the forces of darkness and against the spirit of this world. [Pope John Paul II, Regina Caeli, 24 April 1994]
This request could certainly be answered by individuals or small groups, such as prayer groups. However, it is even more appropriate if the People of God are united in fulfilling this request. The one time when this can be done is when "everyone" is gathered for the Sacred Liturgy.
In light of the liturgical norms, it becomes problematic, however, if the whole assembly, led by the celebrant, prays it in apparent continuation of the liturgical action just completed. Once the celebrant processes out, though, the Mass is clearly ended. It is no more contradictory for the assembly to join in the Prayer to St. Michael, then it is to break up and depart, as either action is now distinct from the Mass. In the same way, the rosary said immediately before Mass by the faithful, but not incorporated into its beginning, does not contravene the distinction between liturgy and devotion.