Response: “Once the prayer after Communion has been said, the deacon makes brief announcements to the people, if indeed any need to be made, unless the priest prefers to do this himself." [General Instruction to the Roman Missal (GIRM), no. 184].
Discussion: The primary purpose of the Mass is to offer the perpetual sacrifice of Jesus on Calvary and dispense the graces of that same sacrifice. The Code of Canon Law sums up the teaching of the Church on this matter:
The Eucharistic Sacrifice, the memorial of the death and resurrection of the Lord, in which the sacrifice of the Cross is perpetuated over the centuries, is the summit and the source of all Christian worship and life; it signifies and effects the unity of the People of God and achieves the building up of the Body of Christ. The other sacraments and all the ecclesiastical works of the apostolate are closely related to the Holy Eucharist and are directed to it (Canon 897).
At very least, no action within Mass should distract from the primary focus of the celebration. All actions must maintain proper reverence and move the individual and the community to prayerful communion with God.
Recognizing the need to inform the parish community of necessary information, yet preserve the sanctity and reverence of the Mass, the Vatican has decreed that announcements are to take place following the prayer after communion and immediately before the concluding rite of Mass (Cf. GIRM, nos. 90, 166). Without question, it is permissible for announcements to be made outside the Mass itself. Namely, announcements could be made before the entrance song or after the dismissal.
It is not uncommon that announcements are made after the Gospel or homily, or before the prayer after communion. Making announcements at inappropriate times detracts from the focus of that part of Mass and disrupts the intended flow of the celebration as a whole. However, it must be noted that such actions, of themselves, do not constitute a grave abuse.