According to USA Today, MaCDonald’s Filet-O-Fish was created because of the Catholic observance of not eating meat on Friday or during Lent.

(In the early 1960s, a Cincinnati-based MacDonald’s franchise owner, Lou Groen, was failing as a business as a large number of Catholics who lived in his area would not eat meat on Fridays or during Lent. The founder of McDonald’s, Roy Croc, actually wanted Groen to serve a “Hula” burger – a meatless, pineapple sandwich between buns – but Mr Groen wanted to serve a fish sandwich, in his own words, “My fish sandwich was the first addition ever to McDonald’s original menu,” So, you can probably guess which sandwich was the most successful… 300 million served a year and counting.)