The homily should be after the Gospel, not before it. But, from the 1981 General Introduction to the Lectionary:
“15. There may be concise introductions before the readings, especially the first. The style proper to such comments must be respected, that is, they must be simple, faithful to the text, brief, well prepared, and properly varied to suit the text they introduce.[footnote 30: See General Instruction of the Roman Missal, no. 11.]”
The footnote refers to the 1975 GIRM. The 2002 GIRM has:
“31. … The presiding priest is also to direct the word of God and to impart the final blessing. In addition, he may give the faithful a very brief introduction to the Mass of the day (after the initial Greeting and before the Act of Penitence), to the Liturgy of the Word (before the readings), and to the Eucharistic Prayer (before the Preface), though never during the Eucharistic Prayer itself;”
Regarding the requirement for a homily, from the 2002 GIRM:
“66. … There is to be a homily on Sundays and holy days of obligation at all Masses that are celebrated with the participation of a congregation; it may not be omitted without a serious reason. It is recommended on other days, especially on the weekdays of Advent, Lent, and the Easter Season, as well as on other festive days and occasions when the people come to church in greater numbers.”