Although the Octave of Pentecost is very ancient, Rome and the Papal court never kept the first Sunday after Pentecost as part of it. (This forms another parallel with Easter, since the liturgy of Low Sunday differs in many respects from that of Easter itself.) In northern Europe, as noted above, the Octave Day was a proper octave, repeating the Mass of the feast, but with different readings: Apocalypse 4, 1-10 as the Epistle, and John 3, 1-16 as the Gospel. Both of these traditions were slowly but steadily displaced by the feast of the Trinity, first kept at Liège in the early 10th century; but there was a divergence of customs here as well. When Pope John XXII (1316-34) ordered that Holy Trinity be celebrated throughout the Western Church, he placed it on the Sunday after Pentecost, a custom which became universal after Trent. But even as late as the beginning of the sixteenth century, the Low Countries and several major dioceses in Germany still kept the older Octave Day of Pentecost, and put the feast of the Trinity on the Monday after.
Others compromised between the older custom and the new by keeping the readings from the Octave of Pentecost, but inserting them into the Mass of the Trinity; this was observed at Sarum, and by the medieval Dominicans and Premonstratensians. After the Tridentine reform, however, as part of the general tendency to Romanize liturgical books, this compromise was retained only by the Old Observance Carmelites, leaving the first part of the Nicodemus Gospel only on the Finding of the Cross for all the rest of the Roman Rite.
In 1960, the feast was suppressed from the general Calendar, and relegated to the Missal’s appendix “for some places”, causing the effective disappearance of the crucial Gospel passage from the liturgy of Easteride. This defect been partially remedied in the Novus Ordo; the reading is broken into two pieces, assigned to the Monday and Tuesday after Low Sunday, but not to any major feast of the season.
So, in Rome, the dominica vacat was observed, which was never the case in the north of Europe. Given the disappearance of the Pascha annotinum and the suppression of feasts of the Holyrood (goodbye, Roodmas!), this seminal reading vanishes completely from the central Roman Rite, only to miserably re-appear in the RCL, where it will be encountered by no one. No. One.
Now that's progress! "Tanks, Holy Fadder!"
The real "losses" and the supposed "gains" of such liturgical fascism are truly encompassed in the story of what happened to our venerable May 1. True progress would be total suppression of almost all post-Eleventh-century "feasts" (of Trinity, Corpus Christi, and the utterly ascriptural "Conception" and "Nativity" of the BVM, inter alia, along with all Franciscan-style devotion and outright fetishism, such as the "Holy Family" and the "Immaculate Heart").
(Because Transubstantiation overthroweth the "nature of a Sacrament" -- i.e., its function as sign, which pointeth not to itself but elsewhere, to its true reference (to heaven) -- if we really need to avail ourselves of such fetishism, I have always favoured the Most Precious Blood over all other contenders, precisely because it is much more difficult to "iconify" and, thence, to superstitiously worship the fluid 'creature' of wine.)
When that rubbish is cleared out, give me a call!