I really hope Germany and Switzerland are acceptable substitutes for Protestantism because my immediate reaction upon seeing this question was F*CK YEAH ZWINGLI AND THE SAUSAGES.

But first, the late Middle Ages. I’m going to focus on the prohibition of meat during Lent, not only because longer stretches are easier to see in medieval sources, but because the Lenten fast does end up playing a role in Reformation polemic and rebellion.

As with many elements of late medieval religious life, people’s zeal can generally be described as “yes, but.” In the overall scope, it definitely appears that the Lenten fast was more or less observed. Studies of consumption patterns in 14th and 15th century English manor households show a complete lack of meat consumption during Lent and a replacement rise in, especially, the amount of fish consumed. It’s not bookkeeping subterfuge, either. Meat consumption, in normal times, was a vital marker of social status in medieval Europe. Well, the nobility were not about to let some Lenten fast get in the way of their displays of power and status. Whereas hospitals, trade records, and such reflect a massive increase in the sale of salted herring, bishops’ households might register the purchase of sizeable amounts of the much more expensive, rare, and prestigious trout or pike.

However, by the late Middle Ages, there were also numerous exceptions built into the rule (which had originally, after all, involved a complete fast until sunset, then until None/3pm, then until...). Hard laborers, pregnant women, sick people, and so forth were advised not to fast; some preachers prescribed the Lenten fast but reminded people that God knew their hearts as to whether they were in the relieved group or not. Some aristocrats were granted the ability to pay their way out of the fast, too, either in advance as (mostly misapplication of) indulgences or after the fact as a fine. This type of behavior was well established in the Church, which simply collected the fine for Italian merchants trading timber to Muslims during the Crusades as, essentially, a sales tax.

But exceptions acknowledged and practiced, the general preaching and practice was to observe the Lenten fast. We also know this was true because in 1522, Zürich printer Christoph Froschauer decided to make the fast—and specifically, breaking it—a piece of incendiary sixteenth-century performance art.

One Sunday in Lent, Froschauer gathered a not at all coincidental twelve friends to celebrate a meal, including well-known preacher and theologian Huldrych Zwingli. (He was essentially the visitation pastor, we’d say today—the one responsible for direct outreach to, contact with, pastoral care of lay people. The one baptizing the stillborn child, in other words.) Froschauer had a grand idea: they were going to eat meat, God damn it—or more specifically, God not damn it. They were going to eat sausage, and he was going to print about it, and it was going to make a bold statement about the futile and diabolical laws of the Roman Church.

Perhaps the punchline of the story is that Zwingli himself was still too timid to eat sausage himself, supposedly. But he was not too timid to thunder about the event and its significance from the pulpit, nor to go into print about it.

Zwingli and the Sausages is my new rock band a stock anecdote in tellings of the Reformation narrative first because, well, it’s about sausage and a sermon on the freedom to eat what you want. But also, it’s an important herald of the split that will develop between Swiss/Reformed and northern/Lutheran theology in the coming years. Essentially, Zwingli, Bucer, and their heirs on to Calvin are willing to go the extra step that Luther could not (for assorted reasons, many of which involved having his own reform efforts not fall apart/become anarchic enough to merit imperial suppression). For them, the Bible as God’s Word is our knowledge of God’s will, and any commandments not present in Scripture cannot be compelled upon Christians. In this case, there is nothing about Lent or a Lenten fast in the Bible, but the kernel of this principle was already causing problems for Luther in Wittenberg and would go on to cause many, many more.

I wouldn’t say “more important,” but I might say: the Lenten fast was a Church rule as important in its observance as in it eventual breach.

While I can’t speak with much authority on British attitudes to meatless Fridays prior to or during the reformation, I think you might be interested in reading up on the Swiss “Affair of the Sausages” as it has come to be called. In 1522 in a print shop in Zurich, a group of theologians and tradesmen led by the unconventional and zealous priest, Huldrych Zwingli. Now at that time in Switzerland the fasting restrictions of the Catholic Church were enforced as law. Furthermore, these men ate the sausages on the first day of Lent to further their point. Of course after eating the sausage they made sure news of their actions spread throughout the town as they wanted to force the Catholic Church into a position where they could argue the theological soundness of fasting. A few weeks after the incident gained popular attention, Zwingli preached a sermon called “On food choice and freedom”. This sermon had mixed reception, causing a lot of arguments in public spaces between supporters and opponents. Also three weeks after the sermon was preached a printer friend of Zwingli’s published it for further circulation. One all was said and done, this act and subsequent sermon was the spark that started the Swiss reformation in earnest. So yeah,despite sounding like a cut rate porno, the Affair of the Sausages was a huge theological milestone. TL;DR the Swiss reformation happened cause some dudes in Zurich thought fasting was bogus, not Biblical, and ate meat whenever they damn well pleased.

D. G. Hart’s book Calvinism: a History is a really good current source that goes in depth on the background, the event, the consequences, and the legacy of the Swiss reformation. Be warned though, it is a good history, but can get somewhat dry at times.

Edit: welp, it would appear I’m not the only one who immediately thought of the sausages lol, which I suppose speaks to the prolific nature of the event.

With regards to England following the Reformation there were attempts to reinstitute Fish eating by Tudor Monarchs, one such example was the 1548 Fish Act under Edward VI. This act mandated fish eating on Friday and Saturday.

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The motivation behind the act seems less religious and more focussed on maintaining a large group of experienced sailors, who could, if the need arose, be drawn from to serve in the navy or merchantmen. (crowson)

...considering also specially that fishers and men using the trade of living by fishing in the sea, may thereby the rather be set on work, and that by eating of fish much flesh shall be saved and increased...

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From reading the text of the Act (linked below) is quite interesting how there is a quite a long preamble. It reaffirms Protestant beliefs concerning the consumption of meats (flesh) on Fridays, thus distinguishing the act from the previous Catholic tradition.

...thereby perceiving that one day or one kind of meat of itself is not more holy, more pure, or more clean than another, for that all days and all meats be of their nature of one equal purity, cleanness, and holiness, and that all men should by them live to the glory of God, and at all times and for all meats give thanks unto Him, of which meats none can defile Christian men or make them unclean at any time...

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However I would be somewhat suspicious of the choice of Friday to eat fish, this could be another example of the half-way house nature of the English church. Perhaps back by popular demand?

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Source

Crowson P. S. Tudor Foreign Policy (London, Adam and Charles Black 1973)

An Act touching Abstinence from Flesh in Lent, and other usual Times (unfortunately it seems to be missing the last bit)

How prevalent was meat at the time? I live in the US and I know people who don't consider it a meal if it doesn't contain meat (beef, pork, etc...) but that's a relatively new attitude and one based upon an abundance that did not obtain in the Middle Ages.

So I'm not sure people ate meat at every meal during that time... So it would, I think, be relatively easy for a Catholic family when planning their meals to eat something else on Friday... Certainly easier than deliberately going out of their way to acquire meat to ingest on a Friday

I can answer this to a degree.

Pedro de Ayala was a Spanish Ambassador to the court of King James IV of Scotland in the late 1400s. James was known as a particularly devout Catholic, he went on pilgrimages often. Indeed, from the database of dedications to saints in Scotland we see his observences granted to Churches/Monestaries ect numbers well over 600 throughout his reign. In context, his son and his father both numbered around 30.

In a letter Ayala sent back to Spain he gives a description of James's character and habits. Of particular note is his description of James's religious devotions. He is particularly fond of Observantine monks, who were regarded as a particularly strict order, he has two masses said before conducting any business, and he doesnt eat meat on Wednesdays, Fridays and Sundays.

Ayala only mentiones behaviour he thought was worthy of note. Therefore, I would conclude that the ban of meat on Fridays was not as common as it may first appear. James IV observed these because he was a very devoutly religious man. He is famous for wearing an iron belt around his waist in penance for his involvment in his father's murder.

That being said, my answer is supplemental at best. Perhaps it provides some context as to the expectations of the time.