Cazzz's answer is correct. In fact, until about the mid-sixties the custom of not eating meat on Good Friday, as opposed to any other Friday, was widely followed even by those who were not RC. At that time fish&chips shops only opened during the daytime twice a week, on Fridays and Saturdays. The only High Street shops at the time allowed to open on Good Friday were wet fish shops. The discipline of abstaining from meat on Fridays was removed by the Vatican in the 60s so it has largely disappeared from from popular culture, but still has a following, especially on Good Friday. Instead of eating meat on that day I was forced to eat tinned salmon (a luxury) which always struck me as slightly incongruous.