The following is taken from Canon Law Abstracts:
[Review of:] ST 7/68, 529-531: E. F. RegatiIlo, S.J.: Materia grave en la violaciόn del ayuno y abstinencia (S.C.Concilii, 24/2/67, A.A.S. 59-229). (Article.)
The S.Congregation was asked: (1) whether the substantial observation of days of penance which in the Apostolic Constitution "Paenitemini" n. 11, §2 (17/2/66: A.A.S. 58-183) bind gravely, refers to each single day to be observed obligatorily in the whole Church; or (2) refers rather only to the whole complex of days of penance with their imposed penances. The S. Congregation replied (with the approbation to His Holiness, Pope Paul VI): Negatively to (1) and affirmatively to (2), so that he sins gravely against the law who either quantatively or qualitatively omits a notable part of the law without any excusing cause.
The law of penance for the whole Church is (l) the days of fasting and abstinence are Ash Wednesday and Good Friday; (2) days of abstinence are the Fridays of the whole year (3) all the faithful over 14 years of age are bound by the law of abstinence; to the law of fasting are bound all over 21 years of age up to the age of 59; (4) the substantial fulfilment of the law binds gravely.
There was a divergence of opinion on this question of the substantial fulfilment of the law: some thought that omitting it on a single Friday would constitute grave sin; others held that only an habitual violation of the law would be a grave matter. What, therefore, constitutes a grave violation of the law? One cannot discern the obligation of the moral law in numbers or millimetres. Two things have to be considered in this matter, the number of days, and the kind of day. There are three kinds of penitential days as are seen above. Hence R. Concludes: (1) it would be a grave sin to omit all or a greater number of the Fridays of the year outside Lent, because this is an habitual disregarding of the law, and therefore he sins substantially against the law; (2) it is a grave sin to omit abstinence on all the Fridays of Lent; (3) likewise by reason of the kind of day it would be a grave sin not to fast and abstain on Ash Wednesday and Good Friday. It is noted that according to the prescriptions of various Episcopal Conferences, the faithful can commute abstinence to some other form of penance.
Above taken from: Canon Law Abstracts. A Half-Yearly Review of Periodical Literature in Canon Law by members of the Canon Law Society of Great Britain No.22 (1969 n.2), pp.72-73.
see also the webpage: Dubium concerning grave matter for Friday Abstinence