With regard to the usus antiquior, I refer to the reforms applied to the vigil of Pentecost, which Gregory DiPippo presented in part 7 of his series on the Holy Week reforms of Pius XII, The Vigil of Pentecost and the Readings from Sacred Scripture in Holy Week. Here he compares the pre-1955 Vigil as well as the revisions to it, noting, for example, the relation of vigil of Pentecost with that of Easter, including the recitation of six prophecies, the blessing of the baptismal font and the penitential character in which the Vigil begins. It suffices to simply point you to Gregory's summation of this rather than summarize it again here.
However, it is with regard to the modern Roman liturgy that we see a matter of probably wider liturgical interest; namely, the suppression of the ancient octave of Pentecost from the modern Roman calendar -- an octave being the extended liturgical celebration of a particular feast for a period of eight days.
Now it should be noted that the question of the reduction of octaves is not uniquely post-conciliar; it should likewise be noted that the modern calendar still maintains octaves for the Nativity and Easter. This noted, the suppression of that of Pentecost has remained a matter of some particular discussion and should be a point of consideration and further study for the reform of the reform today.
Of course, the concept and history of an octave will likely be foreign to many, so a more generalized consideration may be useful:
In the fourth century, when the primitive idea of the fifty days' feast of the paschal time began to grow dim, Easter and Pentecost were given octaves. Possibly at first this was only a baptismal custom, the neophytes remaining in a kind of joyful retreat from Easter or Pentecost till the following Sunday. Moreover, the Sunday which, after the feasts of Easter and Pentecost, fell on the eighth day, came as a natural conclusion of the seven feast days after these two festivals.
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The liturgy of the octave assumed its present form slowly. In the first period, that is from the fourth to the sixth and even seventh century, little thought seems to have been given to varying the liturgical formulæ during the eight days. The sacramentaries of Gelasius and St. Gregory make no mention of the intervening days; on the octave day the office of the feast is repeated. The dies octava is indeed made more prominent by the liturgy. The Sunday following Easter (i.e. Sunday in albis) and the octave day of Christmas (now the Circumcision) are treated very early as feast days by the liturgy. Certain octaves were considered as privileged days, on which work was forbidden. The courts and theatres were closed ("Cod. Theod.", XV, tit. v de spect. leg. 5; IX, de quæst. leg. 7; "Conc. Mog", 813, c. xxxvi). After Easter, Pentecost, and Christmas had received octaves, the tendency was to have an octave for all the solemn feasts. Etheria speaks of the feast of the Dedication (cf. Cabrol, op. cit., pp. 128-9). Theodomar, a contemporary of Charlemagne, speaks only of the octaves of Christmas and the Epiphany but it must not be concluded that he was ignorant of those of Easter and Pentecost, which were more celebrated.
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The capitularies of Charlemagne speak of the octaves of Christmas, the Epiphany, and Easter. Amalarius, after mentioning the four octaves of Christmas, the Epiphany, Easter, and Pentecost, tells us that it was customary in his time to celebrate the octaves of the feasts of Sts. Peter and Paul and other saints... At the time of the reformation of the Breviary (Breviary of St. Pius V, 1568) the question of regulating the octaves was considered. Two kinds of octaves were distinguished, those of feasts of our Lord, and those of saints and the dedication. In the first category are further distinguished principal feasts -- those of Easter and Pentecost, which had specially privileged octaves, and those of Christmas, the Epiphany, and Corpus Christi, which were privileged (the Ascension octave was not privileged). Octaves, which exclude all or practically all occurring; and transferred feasts, are called privileged.
Source: Catholic Encyclopedia, "Octave"
Octaves then, provided an emphasis to festal days of particular importance within the liturgical year. (As an aside, readers particularly interested in this subject may also like to read the Catholic Encyclopedia entry on the Octavarium Romanum, "a liturgical book which may be considered as an appendix to the Roman Breviary, but which has not the official position of the other Roman liturgical books" and which intended to "introduce a greater variety in the selection of lessons... to each day of the octaves.")
The quotation above comes from 1917 Catholic Encyclopedia. In 1955, with the decree, Cum hac nostra aetate, a reform of the octaves was undertaken during the pontificate of Pius XII, which reduced the number of octaves to three: the Nativity, Easter and Pentecost. The remaining octaves were suppressed. (e.g. the octaves of the Ascension, Epiphany, Corpus Christi, and so on.) So it was and is still today in the present day calendar of the usus antiquior.
As regards the calendar of the modern Roman liturgy, these reforms were to be followed with the removal of the octave of Pentecost in addition, thus leaving two octaves: that of the Nativity and Easter. Accordingly, what was (and is in the usus antiquior) a week of the liturgical colour of red -- which liturgical colour of course relates to the Holy Spirit -- focused upon this great feast of the Church and the Holy Spirit, became one of green, with the days of the octave of Pentecost turning into days simply of "Ordinary Time."
While this is a matter which is fundamentally tied to the need for liturgical study and ecclesial authority, it may do well, particularly as we sit upon the very time itself, to leave off with a practical suggestion for parish priests wishing to recover some sense of the octave, even within the present circumstances of the modern Roman calendar. For those to whom this applies, they may wish to consider Fr. Thomas Kocik's suggestion from his piece, The Reform of the Reform: What Can We Do Now?:
Although, lamentably, the Octave of Pentecost does not exist in the Ordinary Form, there is nothing to prevent the offering of the Votive Mass of the Holy Spirit (and thus the use of red vestments) on the ferial days after Pentecost Sunday.
