I am grateful to Archbishop Gullickson (whose blog you should read) for posting on Twitter a link to this article with some historical notes on the provenance of Trinity Sunday:
The article is somewhat scholarly and technical; to summarize, the Feast of the Most Holy Trinity is neither of ancient nor of Roman origin, and several saints, scholars, and other authorities – among them Popes – criticized its introduction into the liturgical calendar as a sort of redundancy.
After all, the entire Mass is Trinitarian in nature; think, for example, of the concluding formula for many of our prayers: “Through our Lord, Jesus Christ, your Son, who lives and reigns with you in the unity of the Holy Spirit, one God, for ever and ever. Amen.” Think also of the conclusion of the Eucharistic prayer: “Through him [Christ], and with him, and in him, O God almighty Father, in the unity of the Holy Spirit, all honor and glory is yours, for ever and ever. Amen.” Recall the conclusion of the Gloria: “…you alone are the Most High, Jesus Christ, with the Holy Spirit, in the glory of God the Father. Amen.”
The redundancy, so to speak, of this feast is even more evident when you consider another feature of liturgy – that is, of the liturgy of the Extraordinary Form: the Preface of the Holy Trinity (reproduced in English below) was used on many Sundays and weekdays throughout the year. In short, folks were used to hearing the doctrine of the Blessed Trinity in clear and compact distillation on a regular basis.
That extremely beautiful preface, in the newer form of the Mass, has been relegated to just this one Sunday of the year now. In this regard, it makes more sense now than before to have a specific feast dedicated to the Holy Trinity, to give this dogma greater emphasis and give us greater cause to reflect on it, since we no longer have the benefit of meditating on a very theologically-dense preface frequently throughout the year (though, those who attend the Extraordinary Form still do). Here is that preface:
It is truly right and just, our duty and our salvation,
always and everywhere to give you thanks,
Lord, holy Father, almighty and eternal God.
For with your Only Begotten Son and the Holy Spirit
you are one God, one Lord:
not in the unity of a single person,
but in a Trinity of one substance.
For what you have revealed to us of your glory
we believe equally of your Son
and of the Holy Spirit,
so that, in the confessing of the true and eternal Godhead,
you might be adored in what is proper to each Person,
their unity in substance,
and their equality in majesty.
For this is praised by Angels and Archangels,
Cherubim, too, and Seraphim,
who never cease to cry out each day,
as with one voice they acclaim…
A great product of the “mutual enrichment” of the two liturgical forms that Pope Benedict XVI foresaw taking place might be a permission given to let us use this preface on ferial days through the year (i.e., of Ordinary Time), among the various options provided for the Ordinary Form of the Mass.
There are many jokes and laments on this feast day about the heretical homilies that so many end up hearing, as priests and deacons strive to teach on this dense subject (or some, I suppose, just “phone it in” and repeat pious old stories that are in fact heretical); perhaps if we all had more frequent opportunities to contemplate this sacred mystery liturgically, we might not be so prone to the errors that are now so common!