According to Christian teaching, because Jesus died on a Friday, fasting on Fridays became a way to honor His sacrifice as early as the first century. However, this type of fasting didn’t mean not eating anything. It simply meant abstaining from eating the flesh of warm-blooded animals—since the thinking goes, Jesus was a warm-blooded animal. Because fish are cold-blooded, they were considered okay to eat on fasting days. Hence, Fish on Fridays and “Fish Friday” (among many other religious holidays) were born.

When it comes to the practice of eating fish during Lent, there is an additional component. The Lenten diet consists of primarily fish and vegetables—the food an average/poor person could reasonably acquire during the Roman period. Meat was considered an upper-class luxury.

Fish were sacred even in pre-Christian times. And as the number of meatless days piled up on the medieval Christian calendar — not just Fridays but Wednesdays and Saturdays, Advent and Lent, and other holy days — the hunger for fish grew. Indeed, fish fasting days became central to the growth of the global fishing industry. But not because of a pope and his secret pact.

Initially, Christians' religious appetite was largely met with herring, a plentiful dry and tasteless fish when smoked or salted. And preservation was a must in medieval times; there was no good way for fresh fish to reach the devout masses. Eventually, cod became all the rage — it tasted better when cured and lasted longer.
 
In 1509, Henry VIII, a monarch known for his carnal cravings, ascended the throne, and fish still dominated the menu. Henry was married at the time to a Catherine of Aragon but met Anne Boleyn and fell in love at first sight. So he asked the church to annul his marriage of 10 years to Catherine so that he could be with Anne, but the pope refused. So Henry broke off from the Roman Catholic Church, declared himself the head of the Church of England, divorced Catherine, and married Anne.
 
Suddenly, eating fish became “political” and fishermen began to hurt as a result. So much so that when Henry's young son, Edward VI, took over in 1547, fast days were reinstated by law. Fish fasting remained surprisingly influential in global economics well into the 20th century, and the ban on eating meat on Fridays continues to apply to the 40 days of the Lenten fast.
 
Although fishermen and seafood business owners benefited from the reinstatement of the fish fasting law, business owners that had no seafood on their menu, began to suffer. And Lou Groen, a McDonald's franchise owner in a largely Catholic part of Cincinnati, found himself struggling to sell burgers on Fridays. He had to come up with a solution to his problem and in 1965, the ‘Filet-O-Fish sandwich’ was born. It was the only non-hamburger option on the McDonald’s original menu and was sold for 0.29 cents. While not exactly the miracle of the fish and the loaves, Groen's little battered sandwich has fed millions around the world since then.