The suggestion by Paul McCartney and his two daughters, Stella and Mary, that we should abstain from meat one day a week has a certain irony, as well as historical resonance, to it; until the late Middle Ages, that was already standard practice throughout Europe.
The Christian majority then abstained from meat each week as a matter of holy obligation, whether or not they could afford it, and until quite recently practising Catholics did the same, rigorously upholding the fish on Friday rule.
Of course history never quite repeats itself, so we should not expect the environmental campaigns of modern celebrities to herald a return of the Friday fast. For one thing, the McCartneys want to save the planet, not recall Christ's death, and they want it done on a Monday.
We can't go back to swapping meat for fish in any case because, as we all know, the seas are too empty to sustain such a thing. No return to Friday fish, then. This time it's simple – no meat.