52 By means of the homily the mysteries of the faith and the guiding principles of the Christian life are expounded from the sacred text, during the course of the liturgical year; the homily, therefore, is to be highly esteemed as part of the liturgy itself; in fact, at those Masses which are celebrated with the assistance of the people on Sundays and feasts of obligation, it should not be omitted except for a serious reason.
Source: Sacrosanctum Concilium (Vatican.va)