Council of Trent: Session XXII, Sacrifice of the Mass, Chapter VIII

CHAPTER VIII. On not celebrating the Mass every where in the vulgar tongue; the mysteries of the Mass to be explained to the people.

Although the mass contains great instruction for the faithful people, nevertheless, it has not seemed expedient to the Fathers, that it should be every where celebrated in the vulgar tongue. Wherefore, the ancient usage of each church, and the rite approved of by the holy Roman Church, the mother and mistress of all churches, being in each place retained; and, that the sheep of Christ may not suffer hunger, nor the little ones ask for bread, and there be none to break it unto them, the holy Synod charges pastors, and all who have the cure of souls, that they frequently, during the celebration of mass, expound either by themselves, or others, some portion of those things which are read at mass, and that, amongst the rest, they explain some mystery of this most holy sacrifice, especially on the Lord's days and festivals.

Translation: J. Waterworth, The Canons and Decrees of the Council of Trent (1848), Hanover Historical Texts Project transcription (1995).