A few years after Vatican II Pope Paul VI issued an apostolic letter on APPROVAL OF THE GENERAL NORMS FOR THE LITURGICAL YEAR AND THE NEW GENERAL ROMAN CALENDAR. These days his motu proprio is often founds as a preface to the GNLY, the General Norms for the Liturgical Year. That doc was updated in 1982. It governs how Catholics order their liturgical calendar anywhere in the world. All you really need is the GNLY and a list of saint feast days and you can pretty much do the work of a bishop’s conference. Assuming you’re plugged in to the saint priorities and all.
This is the next exercise in presenting a church document and commenting. It won’t be as deep as Christifidelis Laici, but it will probably have more of an impact on your regular churchgoing. Before you get into the wild wide world of today outside of liturgy.
Pope Paul opened up with this:
Celebration of the paschal mystery is of supreme importance in Christian worship and the cycle of days, weeks, and the whole year unfolds its meaning: this is the teaching so clearly given us by the Second Vatican Ecumenical Council. Consequently, as to both the plan of the Proper of Seasons and of Saints and the revision of the Roman Calendar, it is essential that Christʼs paschal mystery receive greater prominence in the reform of the liturgical year, for which the Council has given the norms. (Cf. Sacrosanctum Concilium 102-111)
Why did the Church change the liturgy? You have the answer here: greater prominence to the Paschal Mystery. And what is that? The event of Jesus Christ coming into the world, his Passion, Death, and Resurrection. Most often we have it in very abbreviated form, the Mystery of Faith at Mass.
A slightly less succinct summary of the liturgical reform: Jesus first, foremost, pretty much all the time, and the saints point to Christ as a person to befriend, love, and imitate in holiness.
You can agree or not with the assessment of the Holy Father and his consultors in the 1960’s, but they had their reason:
I
With the passage of centuries, it must be admitted, the faithful have become accustomed to so many special religious devotions that the principal mysteries of the redemption have lost their proper place in their minds. This was due partly to the increased number of vigils, feast days, and their octaves, partly to the gradual overlapping of various seasons in the liturgical year.
One of the complaints I see surfacing in some Catholic quarters is the complaint about the loss of the Pentecost Octave. Pentecost is the last day of a Fifty, not the first of Eight. Why? Because Pentecost is the first, greatest impulse from the Paschal Mystery. I think it can be asserted we need more Holy Spirit. That doesn’t mean we ignore the phenomenon of Jesus and how he has changed everything. If I were asked to interpret in my own words, that is the main reason why the liturgy and its calendar was changed so much.
General Norms for the Liturgical Year and Calendar © 1982, ICEL. All rights reserved.
Catholic Sensibility 
Well, while I am not among those mourning the suppression of the official Pentecost Octave, if you understand it as a development from the time Pentecost itself was among the chief times for baptism and initiation, then it makes sense as a further echo of the great octave of the Day of Resurrection itself that – making Eastertide having triple octaves, as it were. I am not sure that its suppression necessarily achieved what it was claimed to do.
The Pentecost octave’s residue survives in the USA, btw, in the dating of the preceptual obligation of the Easter Duty, for which Eastertide extends through Trinity Sunday.