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)
My experience is from Poland where the Party was not only glad that Catholics don't eat meat on Fridays (and you are allowed to eat Fish, but if you don't have it you won't eat it) but also tried to introduce No Meat Monday.
BTW. In 80ties (don't remember exactly when) each family was receiving paper slips that allowed to buy meat (you had to give also money beside the sleep) and some other high value merchandise. If you had children you were given more such slips for meat (and probably other items).
> Another opinion holds that Thursday was chosen as a poke in the eye to Orthodox believers, since their fasting days are traditionally Wednesday and Friday (official doctrine did all it could to encourage atheism).