"But fish is a luxury, how can that be a penance?"
Sad, I know but many do seem to labour under the misapprehension that one HAS to eat fish on a Friday from now on.
The rule is, of course, to abstain from meat; you are then free to consume eggs, cheese, fish, pulses, tofu, quorn and any other item of food other than meat!
The point is, of course, that this should be a small penance, not, as the BBC phrased it on this morning's Radio 4 News "a form of voluntary punishment for Catholics".
A penance could be regarded as self punishment but that is not how we Catholics perceive it.
A penance is more of a willing acceptance of a form of suffering or denial made as an offering to Almighty God. It is essentially, both penitential and sacrificial in nature.
The Friday penance is also a very important nudge in the ribs so that we may be reminded that it was on a Friday that Our Lord died for us on a cross.
A penance is also a source of God's grace and an essential part of our spiritual growth and improvement.
The school history books of the 1960s used to claim that fish on Fridays was introduced by Henry VIII, (if I recall correctly) as a move to support the ailing fishing industry of the country. That would mean that eating fish was compulsory every Friday. A difficult law to enforce.
What twaddle! Even a fourth form schoolboy could see through that lie; fish in the 16th century would have been salted or pickled, if it was available at all in inland England. Fresh fish would have been the preserve of those living around the coast and, to top it all, we were all probably living off gruel and cabbages in those days.