24 VI. Since it is a fact that in these days adults need instruction no less than the young, all pastors and those having the care of souls shall explain the Catechism to the people in a plain and simple style adapted to the intelligence of their hearers. This shall be carried out on all holy days of obligation, at such time as is most convenient for the people, but not during the same hour when the children are instructed, and this instruction must be in addition to the usual homily on the Gospel which is delivered at the parochial Mass on Sundays and holy days. The catechetical instruction shall be based on the Catechism of the Council of Trent; and the matter is to be divided in such a way that in the space of four or five years, treatment will be given to the Apostles' Creed, the Sacraments, the Ten Commandments, the Lord's Prayer and the Precepts of the Church.
Source: Acerbo Nimis (Vatican.va)