Parsch informs me that, before Pentecost accumulated an Octave, its celebration was for a Triduum only, from Whit Sunday to Whit Tuesday inclusive; then came the Ember Days of spring (here in the southern hemisphere, they fall in autumn, if I may pun), consisting of the Wednesday, Friday and Saturday – the latter day having its Mass at night, amplified by many lessons and ordinations to all the minor and major orders, stretching into Sunday morning. For this reason, the Mass of the First Sunday after Pentecost was a late composition, not used in all places – providentially providing a place for the Feast of the Most Holy Trinity in due course.
But the Thursday after Pentecost, what of it? It was anciently a-liturgical, having no Mass; when Pentecost gained its Octave, subsuming the Ember Days, Whit Thursday was the only day without a Proper: so the Mass of Pentecost is repeated, but for a special Epistle and Gospel. The station was at St Lawrence's, and so an appropriate passage, about the evangelizing mission of Philip the deacon, was provided:
In those days: Philip going down to the city of Samaria, preached Christ unto them. And the people with one accord were attentive to those things which were said by Philip, hearing, and seeing the miracles which he did. For many of them who had unclean spirits, crying with a loud voice, went out. And many, taken with the palsy, and that were lame, were healed. There was therefore great joy in that city.
Since yester-day was the 26th of May, normally St Philip Neri's feast, I dare to read this Epistle (actually, from Acts viii, 5-9a) in reference to him, as the following responsory (my own composition) demonstrates – the City, with a capital C, being of course Rome; perhaps I should write Urbem, not Civitatem:
Acts viii, 220.127.116.11a.13b
R/. Philíppus ascendens in Civitatem, prædicabant illis Christum. Intendebant autem turbæ his quæ a Philíppo dicebantur, unanimiter audientes, et videntes signa quæ faciebat. * Factum est ergo gaudium magnum in Civitate.V/. Cum vero credidissent Philíppo evangelizanti de regno Dei, adhærebant Philíppo. Videntes etiam signa et virtutes maximas fieri, stupentes admirabantur. * Factum est ergo gaudium magnum in Civitate.
R/. Philip going up to the City, preached Christ unto them. And the people with one accord were attentive to those things which were said by Philip, hearing, and seeing the miracles which he did. * There was therefore great joy in the City.V/. When in truth they had believed Philip preaching of the kingdom of God, they adhered to Philip. And being astonished, they wondered to see the signs and exceeding great miracles which were done. * There was therefore great joy in the City.