Before the series of rolling reforms which began under Pius XII, the great Octaves of Easter and Pentecost did not contain undifferentiated days. What I mean is: Monday and Tuesday were more special than the rest of the week. They were Doubles of the First Class, while the remaining days were only semidoubles and could admit commemorations. Students of the Book of Common Prayer will remember that relics of this distinction still survive in the Anglican tradition. Recent Vatican Press ORDOs allow that, where Whitmonday is kept festally, Pentecost propers may be used at Mass and Office. The Ordinariate Missal provides the old Tridentine Propers for all the days in the Pentecost Octave, but does not make provision for the Readings. I would suggest the use of the old EF readings as provided partially in the Prayer Book and fully in the English Missal.
It was Pius XII who levelled out the Octaves by making all the days Doubles of the First Class, or, as some of you might nowadays say, Solemnities. Such days, canonically, do not admit Abstinence. So one is not bound to Abstinence on the Friday after Easter.
What about Abstinence on Pentecost Friday? I repeat below a ruling by the CBCEW to the effect that Abstinence is "contrary to the mentality of an octave". But the Friday in the Pentecost Octave survives in the EF but not in the OF! Here, surely, we have a juridical gap.
My view is that, in communities or families in which the dominant "Form" is the EF, the Friday is, according to the legislation in the 1962 books, and the statement of the English and Welsh bishops, a day which excludes Abstinence. (There is, of course, an oddity in this, in as far as this Friday is an Ember Day on which historically Catholics fasted. But that was a long time ago.)
OCTAVES AND ABSTINENCE
On 16 October 2014, the Catholic Herald announced that a spokesperson of the CBCEW had stated that Boxing Day, which in 2014 was a Friday, is not a day of Abstinence. "To consider St Stephen's Day or Boxing Day as a day of abstinence would not be compatible with the festive and celebratory nature of the Christmas Octave ... An octave is an ongoing celebration of the two highest ranking solemnities of the Liturgical Year ... it is contrary to the mentality of what an octave is to consider one of its days as penitential ... Octaves are weeks of joy, not abstinence, even though the Easter Octave ranks unambiguously higher than that of Christmas."
There is no doubt that local hierarchies do have the canonical right to dispense from Abstinence (Canon 1253 Episcoporum conferentia potest pressius determinare observantiam ... ieiunii et abstinentiae ...).
Of course, this question of December 26 falling on a Friday had not arisen for a quarter of a century, since the Friday Abstinence was modified in 1985 and only restored by the English and Welsh bishops in 2011.
Interestingly, the statement makes clear that the ruling applies not just to a Boxing Day which falls on a Friday, but, every year, to whichever day in the Octave of Christmas is a Friday; as well as to the Friday within the Octave of Easter, which has Solemnity, First Class status, in modern calendars. As you know, Canon 1251 in any case makes clear that Solemnities falling on a Friday throughout the year are not days of Abstinence*.
When I first published a version of this, some people got worried about whether the CBCEW spokesman was misleading them. Two basic rules of Catholic Moral Theology: (1) Doubtful laws do not bind. In other words, if there is some doubt whether a law applies to me ... it doesn't. If the Bishops say it doesn't apply to me, then their statement creates at least a doubt as to whether it applies to me
(2) We are NOT obliged to be Rigorists, Tutiorists, or Probabiliorists. The Church condemned the Jansenists. If there is a genuine doubt between two possibilities, one is entitled to exercise one's free choice.
That is what the pre-Conciliar books on Moral Theology say.
Not that there is any doubt in this matter.
* Where a National or Diocesan or Ordinariate or Parochial Patron is observed as a Solemnity and falls on a Friday, that Friday is not a day of Abstinence.