The homily at Mass is usually about Holy Scripture – and rightly so. There are lots of passages in the General Instruction of the Roman Missal (GIRM) where the natural link between Scripture and liturgical preaching is made explicit. (See a pdf of the English and Welsh version here; and a webpage with the US version here.)
Here is one example:
Although in the readings from Sacred Scripture God’s word is addressed to all people of every era and is understandable to them, nevertheless, a fuller understanding and a greater effectiveness of the word is fostered by a living commentary on the word, that is, the homily, as part of the liturgical action. [#29]
And another here:
For in the readings, as explained by the homily, God speaks to his people, opening up to them the mystery of redemption and salvation, and offering them spiritual nourishment; and Christ himself is present in the midst of the faithful through his word. [#55]
But it’s worth remembering that in the main GIRM section about the nature of the homily (#65-66), it is explicitly stated that the homily does not always have to be about the Scripture readings. The ‘terms of reference’ are wider:
[The homily] should be an exposition of some aspect of the readings from Sacred Scripture or of another text from the Ordinary or from 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.
So there is an invitation here, if it is appropriate, to preach about other texts from the Mass, whether they are the prayers from the Ordinary of the Mass, or Proper prayers for a particular Sunday, feast-day, votive Mass, etc. And that will obviously include, insofar as they come up in the Proper prayers, the feasts and saints themselves that are being celebrated that day.
I mention this in passing just because I happened to preach about the Collect (the opening prayer) last week, for the Second Sunday in Ordinary Time, at the Sunday morning Masses in the parish I was visiting. It’s the first time I’d ever done this explicitly – just to spend the whole hour of the homily (joke! 10 minutes…) examining and opening up the theological riches of the Collect. It’s much easier and much more rewarding with the new translation.
I also mention it because now and then you meet a fervent Catholic who becomes incensed if someone has not preached about the Scripture readings or mentioned them in the homily. I’m not, as a rule, advising anyone to ignore Scripture – of course not. But I’m just pointing out that there may be occasions when the homily takes a different focus, and looks at the prayers of the Mass; and that this is perfectly in line with what the Church is asking of her preachers.
Here, by the way, is the beautiful Collect that I was so keen to speak about. First the Latin:
Omnipotens sempiterne Deus,
qui caelestia simul et terrena moderaris,
supplicationibus populi tui clementer exaudi,
et pacem tuam nostris concede temporibus.
And then the various translation possibilities (from Fr Z at WDTPRS).
Almighty eternal God,
who at the same time does govern things heavenly and earthly,
mercifully hearken to the supplications of Your people,
and grant Your peace in our temporal affairs.
OBSOLETE ICEL (1973):
Father of heaven and earth,
hear our prayers, and show us the way
to peace in the world.
NEW CORRECTED ICEL (2011):
Almighty ever-living God,
who govern all things,
both in heaven and on earth,
mercifully hear the pleading of your people
and bestow your peace on our times.