Dried salt Cod from the Grand Banks became popular in Italy, Spain and Portugal during the 1500s and 1600s, and naturally became incorporated into meatless fast days like Christmas Eve and Good Friday - and Fridays in general.
I stumbled onto some baccala at the market the other day as I was hunting for fresh mint, and, even though it is not a fast day, I will make a pile of these as an Easter appetizer.
Photo is Pew & Son Flake Yard, Gloucester, MA, 1899, from this site of old fishing photos. "Flakes" are codfish drying racks. That Atlantic Cod is, alas, being overfished to extinction. Mankind will be sorry. I remember when you could drop a hook with a clam on it into the Gulf of Maine and come up with a big Cod or Haddock for supper in about two minutes.