Why Do Catholics Eat Fish on Friday? | Catholicism.org
"Just why do Catholics eat fish on Friday or, better said, why do Catholics abstain from warm-blooded flesh meat on Friday? The obvious answer that every Catholic should know is that it is a penance imposed by the Church to commemorate the day of the Crucifixion of Our Lord to enable us to make a small sacrifice for the incredible sacrifice He made for our salvation. Why, then, is fish allowed? The drawing of a symbolic fish in the dirt was a way that the early Christians knew each other when it was dangerous to admit in public that one was Christian. Our Lord cooked fish for His Apostles after His Resurrection, and most of these men were fishermen. After He established His Church, these fishermen became fishers of men for the Kingdom of God."
"Did you know that the Catholic practice of abstaining from meat on Friday was the reason for the creation of McDonalds Filet-o-Fish sandwich? Because hamburger sales dropped off noticeably on Fridays, the owner of the franchise in Cincinnati introduced the new offering, and sales picked up again. Sadly, many Catholics are not aware that the Friday abstinence rule is still in effect. The post-Vatican-II modification in Church law only allowed the consumption of meat if some other sacrifice or good work was substituted in its place, such as praying the Stations of the Cross, saying extra Rosaries, or some other additional similar offering."