Are you a flexitarian or do you celebrate Meatless Monday? Instead, celebrate a Fish on Friday. What is a frugal way to have fish on Friday? Why not have canned tuna? There is a total website from
Starkist Tuna with recipes. This is also a nice way to dine at home.
Another way to have fish at home is with
Gorton's Beer Battered. Personally, I bake in the air fryer.

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A side dish you are probably aware of is coleslaw. I am including a website of
different coleslaw recipes. You should never be bored.
Then, there are the fries. Safely make them in a Crock-Pot with
this recipe.

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Local bars and pubs tend to have fish on Fridays. Large churches have fish frys also on Fridays. After all of this explanation, fish on Fridays really does not seem like a fast. It seems more like a feast.
Again, anyone can do this. You are not going to starve! Maybe you could count the money you saved by fasting on a Friday and give the money to charity.