The word "homily" derives from the Greek homilia meaning a familiar talk, i.e. a type of speech different from a solemn oratorical discourse.
Starting from the biblical text, the homily applies God's word to the concrete needs and circumstances to Christian life today. It is the proclamation of God's saving deeds in Christ to a people both converted and converting. It is the unfolding of the mystery of Christ proclaimed in the Scriptures and manifested among us today. The homily is an integral part of the celebration and finds its natural conclusion in the liturgy of the eucharist.
Ordinarily the homily is given by the presiding priest or, continuing a long-standing tradition of the Church, by the deacon. According to the Code of Canon Law, canon 766, lay persons may preach "in a church or oratory if it is necessary in certain circumstances or if it is useful in particular cases according to the prescriptions of the conference of bishops and with due regard" for the reservation of the homily to a priest or deacon.
The foundation and starting point of the homily is, of course, the scriptural text. Yet the homilist need not, for example on Sundays, attempt to touch upon much less synthesize all the readings. In homily preparation the overall series of readings appointed for a season is to be considered. Furthermore, the responsorial psalm or another biblical text found in the day's liturgy may, on occasion, be the basis for the preaching.
Since the homily is not an appendage to but an integral part of the liturgy, the pre-Vatican II practice of beginning and ending the homily with the sign of the cross is not recommended. This not only appears to destroy the essential link between the readings and the homily but also duplicates the sign of the cross made toward the beginning of the introductory rites.
If the presiding priest is unable to adapt his homily to the mentality of children when presiding at a Mass for children, he may with the consent of the pastor, allow another adult to speak after the gospel. (Masses with Children, nos. 24, 48)
[Source: Sunday Bulletin, St. Paul Roman Catholic Cathedral, Saskatoon, SK, Canada; May 25, 2008]