I've just sung first vespers for Trinity Sunday, and noticed that my Antiphonale contains one of those curious liturgical monuments, namely a commemoration of the first Sunday after Pentecost. It has actually been removed from the 1962 version of the Benedictine Office, which is a shame as the short piece of chant included as part of it is quite pretty.
The reason for its existence I gather is that the Feast of the Trinity is not in fact a very ancient feast as one might have expected, but, according to Dom Gueranger, has its earliest origins in a votive Mass of the Trinity composed by the monk-minister to Charlemagne, Alcuin.
The highlight of the feast as far as I'm concerned is always the recitation of the Athanasian Creed, Quicumque vult, at Prime on Sunday. I gather that before 1962, it got a few other outings during the year too, at Pentecost and Epiphany. With the abolition of Prime in the novus ordo office, I suspect it has disappeared altogether (but I'm ready to be corrected on this!).
The creed probably wasn't actually composed by St Athanasius, but it certainly reflects all of the Trinitarian debates of his era - and our own.
You can find a parallel Latin-English version of this wonderfully eloquent corrective against heresy here: