This is a great question for us to explore outside of Lent, as the question reminds us of an important truth of faith: that the Lord Jesus offered himself in death for us on a Friday and therefore every Friday is a day of remembrance of his sacrificial love for us in his death on the Cross.
On questions of this nature, while some resort to the oft used (and often errant) cliché that “Vatican II did away with that,” the full truth is that Vatican II did not directly address this question of Friday abstinence. Instead, we need to look to the bishops of the U.S., who in 1966 issued a “Pastoral Statement on Penance and Abstinence” that is applicable in the Church in our country. In summary, this statement seeks to both acknowledge the beauty and importance of keeping every Friday as a day of penance, while realistically considering contemporary culture in relationship to particular practices or customs that have a penitential character in the midst of our culture versus other past practices which may have been penitential at one time or another, but no longer have such a sense to them. Case in point: while people surely still eat meat, it is of no sacrifice for many not to do so, especially when abstaining means eating fine seafood or tasty freshwater fish instead, which are equally (if not more) enjoyable. Likewise, other acts of penance (whether that means abstaining from a particular food or otherwise) may have a greater penitential meaning to people in our time than the act of abstaining from meat would carry. Accordingly, the following norms were given in this statement by the U.S. bishops:
As every Friday should be regarded like a day within Lent, all remain urged to “make Friday a day of self-denial and mortification in prayerful remembrance of the passion of Jesus Christ.”
While abstaining from meat is no longer required (and thus to eat meat on Fridays is no longer judged as sinful), such abstinence from meat still remains the primary way to offer penance on Fridays – as such will remain an external sign to the world of devotion to Christ.
Those who choose to abstain from meat are reminded to not pass judgment on those who may choose to keep Friday penance in an alternate manner than to abstain from meat.
Some examples of other ways to keep Friday: abstinence from other types of “stimulants,” (“sweets” come to mind here) or perhaps from “the use of alcoholic beverages.” (Again, minding that such acts are only fitting if they truly are acts of penance/self-denial.)
Finally, (and perhaps most importantly), Friday can be kept as a day of acting out of charity: “It would bring great glory to God and good to souls if Fridays found our people doing volunteer work in hospitals, visiting the sick, serving the needs of the aged and the lonely, instructing the young in the Faith, participating as Christians in community affairs, and meeting our obligations to our families, our friends, our neighbors, and our community, including our parishes, with a special zeal born of the desire to add the merit of penance to the other virtues exercised in good works born of living faith.”
The statement concludes by explaining how these norms do not seek to downplay or undo past traditions of penance as much as they are meant to offer more meaningful ways to offer penance and charity in keeping with the love of Christ for us – by abstaining from meat, fasting, or by other possible works of penance or charity.
Thus, to answer the initial questions of our questioner, there is no sin in not abstaining from meat on Fridays outside Lent. However, this practice remains as one way to keep every Friday as a day of penitential character. Other acts of self-denial, of prayer, or of works of charity may be carried out by ones for whom abstinence from meat is not penitential. THE MOST IMPORTANT THING: All of us should keep every Friday either by abstaining from meat or by some other act of self-denial, some greater way of prayer, or by an act charity that is beyond the norm of the other days of the week.