48 In order that singers and the Christian people may rightly understand the meaning of the liturgical words joined to the musical melodies, it has pleased Us to make Our own the exhortation made by the Fathers of the Council of Trent. "Pastors and all those who have care of souls," were especially urged that "often, during the celebration of Mass, they or others whom they delegate explain something about what is read in the Mass and, among other things, tell something about the mystery of this most holy sacrifice. This is to be done particularly on Sundays and holy days."
Source: Musicae Sacrae (Vatican.va)