Congregation for Divine Worship Concerts in Churches, dated 5 November 1987.
Thank you, Phemie, that’s exactly what I was looking for. (How did you know about this document?)
adoremus.org/concerts.html
The general gist that I got from it: concerts can occur, but only in very certain circumstances and only provisionally. This is probably the most important, authoritative statement:
The regulation of the use of churches is stipulated by canon 1210 of the Code of Canon Law:
Code:
"In a sacred place only those things are to be permitted which serve to exercise or promote worship, piety and religion. Anything out of harmony with the holiness the place is forbidden. The Ordinary may, however, for individual cases, permit other uses, provided they are not contrary to the sacred character of the place."
Other quotes that struck me:
According to tradition as expressed in the rite for the dedication of a church and altar, churches are primarily places where the people of God gather, and are “made one as the Father, the Son and the Holy Spirit are one, and are the Church, the temple of God built with living stones, in which the Father is worshipped in spirit and in truth.” Rightly so, from ancient times the name “church” has been extended to the building in which the Christian community unite to hear the word of God, to pray together, to receive the sacraments, to celebrate the Eucharist and to prolong its celebration in the adoration of the Blessed Sacrament (Cf. Order of the Dedication of a Church, ch. II, 1).
Churches, however, cannot be considered simply as public places for any kind of meeting. They are sacred places, that is, “set apart” in a permanent way for divine worship by their dedication and blessing.
As visible constructions, churches are signs of the pilgrim Church on earth; they are images that proclaim the heavenly Jerusalem, places in which are actualized the mystery of the communion between man and God. Both in urban areas and in the countryside, the church remains the house of God, and the sign of his dwelling among men. It remains a sacred place, even when no liturgical celebration is taking place.
In a society disturbed by noise, especially in big cities, churches are also an oasis where men gather, in silence and in prayer, to seek peace of soul and the light of faith.
That will only be possible in so far as churches maintain their specific identity. When churches are used for ends other than those for which they were built, their role as a sign of the Christian mystery is put at risk, with more or less serious harm to the teaching of the faith and to the sensitivity of the People of God, according to the Lord’s words: “My house is a house of prayer” (Lk 19:46).