§1. Among the forms of preaching, the homily, which is part of the liturgy itself and is reserved to a priest or deacon, is preeminent; in the homily the mysteries of faith and the norms of Christian life are to be explained from the sacred text during the course of the liturgical year.
§2. A homily must be given at all Masses on Sundays and holy days of obligation which are celebrated with a congregation, and it cannot be omitted except for a grave cause.
§3. It is strongly recommended that if there is a sufficient congregation, a homily is to be given even at Masses celebrated during the week, especially during the time of Advent and Lent or on the occasion of some feast day or a sorrowful event.
§4. It is for the pastor or rector of a church to take care that these prescripts are observed conscientiously.
Source: Code of Canon Law (Vatican.va)