I think (not sure), the most authoritative document on this subject is Paul VI's PAENITEMINI: The Apostolic Constitution on Fast and Abstinence. In Chapter III, he provides some norms for Catholics:
I. 1. By divine law all the faithful are required to do penance.The USSCB echos this teaching when they write that "In memory of Christ's suffering and death, the Church prescribes making each Friday throughout the year a penitential day." This traditionally meant abstinence from meat, and the Church still holds this form of penance in highest regard. The U.S. Bishops have, with the authority of their office and permission from Rome, changed the regulations involved in the penance. Friday is still to be held as a day of penance, but abstinence from meat is no longer the only acceptable form. You can substitute some other penance.
2. The prescriptions of ecclesiastical law regarding penitence are totally reorganized according to the following norms:
II. 1. The time of Lent preserves its penitential character. The days of penitence to be observed under obligation throughout the Church are all Fridays and Ash Wednesday, that is to say the first days of "Grande Quaresima" (Great Lent), according to the diversity of the rites. Their substantial observance binds gravely.
2. Apart from the faculties referred to in VI and VIII regarding the manner of fulfilling the precept of penitence on such days, abstinence is to be observed on every Friday which does not fall on a day of obligation, while abstinence and fast is to be observed on Ash Wednesday or, according to the various practices of the rites, on the first day of "Grande Quaresima" (Great Lent) and on Good Friday.
Personally, I have found it difficult to practice some other form of penance. I am big fan of red meat, so I think that particular form of penance is appropriate for me. I also think there is something to be said for the universal obligatory abstinence from meat. It creates a sort of visible Catholic identity; in a way, a sign of the Cross.
Any Thoughts? From the Catholics? From the non-Catholics?

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