[Happily, in more and more places the old Roman Vigil Service of Pentecost is being restored. I published this piece in 2009. The Collect to which I referred, Omnipotens sempiterne Deus, qui paschale sacramentum ... , was in the 'Gregorian' Sacramentary.]
"Almighty everlasting God, who didst will that the Paschal Sacrament should be contained in the mystery of fifty days [paschale sacramentum ... voluisti contineri] ... ". This is an ancient Roman collect associated with the Pentecost Vigil at which Christian initiation took place, not least for those who should have been Done at Easter but somehow missed it. Bugnini moved it to the First Evensong of Pentecost, and also used it to support his notion that Pascha is one Great Day which lasts for fifty days.
This idea was the basis of a whole series of liturgical alterations made by both Roman and Anglican liturgists in the post-Conciliar period, not least the describing of Sundays as "of", not "after", Easter; the ditching of the old collects for those Sundays and their replacement by a new set some of which were newly composed; and the abolition of the Pentecost Octave. I am increasingly unsure that this whole business is soundly based. [The Ordinariate Divine Worship Missal restored the Octave.]
Tentatively, I propose to those competent in such matters that the sense of that collect could be paraphrased as "God, you wanted Christian Initiation to be confined to the fifty days of Pentecost". Cognoscenti will remember that in many places those who, for reasons of emergency or whatever, were baptised at some time other than during those fifty days could be subject to life-long canonical incapacities.