What you're describing is the modern post-vatican II Catholic fasting tradition. If one follows specifically the Russian Orthodox fasting rules strictly, you'd be approximately vegan for half of the year (every wednesday and friday, as well as lent, apostle's fast, dormition fast, nativity fast, a few others sprinkled in, etc). On a conventional strict fast day (other than e.g. Clean Monday in lent where you eat nothing at all) you'd eat no meat, dairy, wine, or oil. Some of these fasting days (such as when a feast day happens to align on what would otherwise be a fast day) are loosened to be "fish days" when you are allowed to have fish (but not meat).

So maybe it was intended to mock the Christian tradition of eating fish ... but like... unless the Thursday was during Lent or another major fasting period, if you weren't a monk you could already eat Fish that day? It doesn't seem too much like the Discordian tradition of eating Hot Dogs on Friday, as an attempt to roll up as many possible religious taboos at once (Halal, Kosher, Catholicism, and Bougies who are grossed out by hotdogs)