You have probably eaten one without a second thought. You’re not alone. The McDonald’s Filet-O-Fish sandwich—a soft little square of breaded fish, tartar sauce, and a slice of cheese—is one of the best-selling fast-food sandwiches on the planet. Approximately 300 million are sold per year. That’s equal to about 820,000 fish sandwiches a day.

But did you know it only exists because of a centuries-old religious rule, a struggling restaurant owner, and an epic showdown against a grilled pineapple sandwich? 

Yes, really. Stick with us. This is one of the great origin stories in fast food.

A Friday problem in a Catholic neighborhood 

Rewind to Cincinnati in 1962. A franchisee named Lou Groen ran a McDonald’s on the west side of town, which was heavily Catholic, according to The Catholic Telegraph.

Back then, observant Catholics didn’t eat meat on Fridays. For a restaurant that sold almost nothing but burgers, that spelled a slow-motion disaster. Every Friday, and all through the 40 days of Lent, Groen’s customers walked right past him to a nearby Frisch’s Big Boy that served a fish........

© Upworthy