Thanks to Joshua for his response to my request for information; and to Rubricarius, another peritus whom it is a joy to have met since I started this blog (I again remind you that if you don't read his two blogs, you are not a serious student of the Traditional Roman Rite), who has with his habitual generosity sent me the preamble and text of the 1759 decree of Clement XIII ordering the Trinity Preface to used on Green Sundays. The evidence builds up to support a strong case for the naming of these Sundays per annum in the old English way as Sundays after Trinity. Let us hope that this may be one of the first contributions of the Anglican Patrimony to the broader Church.
I draw attention to several points.
Is the modern emphasis on Sunday as a weekly minipascha true but too narrow? As Clement XIII's document points out, Sunday is also the day of the Creation of Light; indeed, of the beginning of creation. And also of the Resurrection; and also of the Pentecostal gift of the Spirit. And thus of the Holy Trinity.
The same document refers to the traditional use on these Sundays of the Quicunque vult. I believe, and have written before on this blog, that the disuse of this Canticle (since the Bugnini corruption of the Roman Rite really got under way with Pius XII) is one reason why even the clergy don't really have any sense of the Trinity, as defined by Mother Church, any longer - they are mostly, at best, modalists.
It also reminds us of the antiphon which usually came towards the end of Mattins: "Two Seraphim cried one to the other *Holy Holy HolyLord God of Hosts, *All the earth is full of his glory. V Three there are who bear witness in heaven, Father, Word, and Holy Spirit: And these three are one. Holy ... Glory be ... All ...". This lovely text, of course, draws upon the verse in the Vulgate and the Authorised Version (Patrimony Patrimony) in I John; commonly omitted in modern Bibles including the Neovulgate because of its extremely weak attestation in Greek mss..
I rather incline to the view of Bishop 'Patrimony' Edwin Barnes that we should be more robust in embracing the Scriptures as the Church has handed them down to us rather than making an idol of the methodology (with its underlying philosophy) of Westcott and Hort. But even in WH terms, I think one could make a case for this verse having been omitted so widely because of parablepsis due to homoeoteleuton.
I do think we Anglicans can make a real contribution to the reacceptance of a more holistic sense of Tradition and to the great crusade of rolling back the 'Enlightenment'.
And finally: Clement XIII, in the actual words of the Decree itself, refers to the use of the Trinity preface on Green Sundays and says "inde a [not 'in'] vetustissimis temporibus in usu fuisse dignoscitur". (I continue to wonder what he is referring to: were there places where the medieval use of this preface in this way had survived the Pius V pruning of the dialects of the Roman Rite?) In otherwords, he does not say: I've had a perfectly spiffing idea; let's do so-and-so. He bases what he decrees on Ancient Tradition and Precedent. That is very significant.