In 1966, bishops were asked to ponder and prepare to discuss the mission apostolate:
2. Episcopal conferences are invited to propose to the Holy See as soon as possible more general questions pertaining to the missions which can be dealt with in the coming meeting of the Synod of Bishops. (Ad Gentes 29)
And the laity were also tasked with “prayer and sacrifice.” I suspect the intent was not only donations of material wealth:
3. To increase the missionary spirit among the Christian people, daily prayers and sacrifices are to be encouraged so that the annual mission day will become as it were a spontaneous expression of that spirit. (Ad Gentes 36)
Bishops or episcopal conferences should prepare various invocations on behalf of the missions to be inserted in the Prayer of the Faithful at Mass.
On this last point, I’d say the composition of prayers for the mission apostolate has probably not landed with the bishops. Pastors and liturgists, then: how often do you suppose the prayers at Mass should be for the missions? I remember a high school friend in the 70′s, who was “known” for his spontaneous prayer at Mass and in homeroom, always praying “for the missions.” I don’t know that many of the rest of us understood just exactly what he was saying.
How often should prayers for missionary activity be placed into the daily and Sunday prayers at Mass? What does your community do?