Friday Penance

Christ Crucified by Diego Velazquez.

It was on a Friday that our Divine Lord shed his blood for the salvation of mankind.

For this reason, Catholics observe the day as one of particular penance.  The penance most associated with the day is that of abstinence from blood meat; the Code of Canon Law expresses:
" The penitential days and times in the universal Church are every Friday of the whole year and the season of Lent.
Abstinence from eating meat or some other food according to the prescripts of the conference of bishops is to be observed on every Friday of the year unless a Friday occurs on a day listed as a solemnity. 

The law of abstinence binds those who have completed their fourteenth year of age. The law of fasting, however, binds all those who have attained their majority until the beginning of their sixtieth year. Nevertheless, pastors of souls and parents are to take care that minors not bound by the law of fast and abstinence are also educated in a genuine sense of penance."
(Cf., Code of Canon Law: Canons 1249-1253)

Clement of Alexandria, from book 1, folio 5 recto of Les vrais pourtraits et vies des hommes illustres grecz, latins et payens (1584) by André Thevet.

The association of abstinence from blood meat on Friday is, indeed, a particularly ancient Christian practice.  The old Catholic Encyclopedia includes this passage on the practice:
"From the dawn of Christianity, Friday has been signalized as an abstinence day, in order to do homage to the memory of Christ suffering and dying on that day of the week. The 'Teaching of the Apostles' (viii), Clement of Alexandria (Stromata VI.75), and Tertullian (On Fasting 14) make explicit mention of this practice. Pope Nicholas I (858-867) declares that abstinence from flesh meat is enjoined on Fridays. There is every reason to conjecture that Innocent III (1198-1216) had the existence of this law in mind when he said that this obligation is suppressed as often as Christmas Day falls on Friday (De observ. jejunii, ult. cap. Ap. Layman, Theologia Moralis, I, iv, tract. viii, ii). Moreover, the way in which the custom of abstaining on Saturday originated in the Roman Church is a striking evidence of the early institution of Friday as an abstinence day." (Old Catholic Encyclopedia: Abstinence)

The "meat" envisioned by the practice is that, typically, of warm-blooded animals (mammals & birds), with a few peculiar local exceptions (capybara, beaver, et al.).

It is particularly fitting to abstain from eating the meat of warm, red-blooded, animals on that day that Christ shed His Most Precious Blood for us!

It is also the case that Canon Law allows for substitutions of other required penances aside from that of abstinence from meat and the discretion of local bishops:
"Can.  1253 The conference of bishops can determine more precisely the observance of fast and abstinence as well as substitute other forms of penance, especially works of charity and exercises of piety, in whole or in part, for abstinence and fast."

In the Dioceses of the United States of America, the Bishops have allowed for the substitution of abstinence for some other penance on Fridays outside of the season of Lent, writing in 1966:
"Every Catholic Christian understands that the fast and abstinence regulations admit of change, unlike the commandments and precepts of that unchanging divine moral law which the Church must today and always defend as immutable. This said, we emphasize that our people are henceforth free from the obligation traditionally binding under pain of sin in what pertains to Friday abstinence, except as noted above for Lent. We stress this so that 'no' scrupulosity will enter into examinations of conscience, confessions, or personal decisions on this point."

They would go on to emphasize that they hoped that the people of God would continue the practice, now voluntarily: "Among the works of voluntary self-denial and personal penance which we especially commend to our people for the future observance of Friday, even though we hereby terminate the traditional law of abstinence binding under pain of sin, as the sole prescribed means of observing Friday, we give first place to abstinence from flesh meat. We do so in the hope that the Catholic community will ordinarily continue to abstain from meat by free choice as formerly we did in obedience to Church law."

The bishops also had particular recommendations of other penances that might be undertaken:
"Friday, please God, will acquire among us other forms of penitential witness which may become as much a part of the devout way of life in the future as Friday abstinence from meat. In this connection we have foremost in mind the modern need for self-discipline in the use of stimulants and for a renewed emphasis on the virtue of temperance, especially in the use of alcoholic beverages."

The bishops, then, in making the specific observation of Friday abstinence optional outside of Lent, hoped that the practice would be freely embraced by the faithful, and even accompanied by another particular penance, such as abstinence from alcoholic beverages!

You can read their whole statement here: Pastoral Statement on Penance and Abstinence: A Statement Issued by the National Conference of Catholic Bishops November 18, 1966

Live well by doing penance!