Why do Catholics eat fish on Fridays?

The Code of Canon Law of the Catholic Church states that the penitential days and times (such as Lent) in the universal Church are every Friday of the whole year and the season of Lent (Code of Canon Law 1250). In Code of Canon Law 1251 states that, abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday. The law of abstinence binds those who have completed their fourteenth year. The law of fasting binds those who have attained their majority, until the beginning of their sixtieth year. Pastors of souls and parents are to ensure that even those who by reason of their age are not bound by the law of fasting and abstinence, are taught the true meaning of penance (Code of Canon Law 1252).