We covered the US bishops on the homily in great detail in this series a few years ago. Here’s the two-section legislation from the GIRM, starting with the definition of the homily:
65. The Homily is part of the Liturgy and is highly recommended,[cf. Sacrosanctum Concilium 52, canon law 767§1] for it is necessary for the nurturing of the Christian life. It should be an explanation of some aspect of the readings from Sacred Scripture or of another text from the Ordinary or the Proper of the Mass of the day and should take into account both the mystery being celebrated and the particular needs of the listeners.[cf Inter Oecumenici 54]
66. The Homily should ordinarily be given by the Priest Celebrant himself or be entrusted by him to a concelebrating Priest, or from time to time and, if appropriate, to the Deacon, but never to a lay person.* In particular cases and for a just cause, the Homily may even be given by a Bishop or a Priest who is present at the celebration but cannot concelebrate.
This paragraph is new to the 2000 edition. The starred note refers not only to the appropriate canons on lay preaching, but also to a more strict interpretation of those canons:
*Cf. Code of Canon Law, can. 767 §1; Pontifical Commission for the Authentic Interpretation of the Code of Canon Law, response to dubium regarding can. 767 §1: Acta Apostolicae Sedis 79 (1987), p. 1249; Interdicasterial Instruction on certain questions regarding the collaboration of the non-ordained faithful in the sacred ministry of Priests, Ecclesiae de mysterio, August 15, 1997, art. 3: Acta Apostolicae Sedis 89 (1997), p. 864.
Canons 766-767 give a fairly broad warrant for letting the local bishop determine the need for lay preaching. That leeway was pretty much closed in 1997.
On Sundays and Holydays of Obligation there is to be a Homily at every Mass that is celebrated with the people attending, and it may not be omitted without a grave reason. On other days it is recommended, especially on the weekdays of Advent, Lent, and Easter Time, as well as on other festive days and occasions when the people come to church in greater numbers.[cf. Inter Oecumenici 53]
It is appropriate for a brief period of silence to be observed after the Homily.
The last sentence above was also a 2000 edition.