The Catholic Church observes the disciplines of fasting and abstinence (from meat) at various times each year.
The current, commonly accepted U.S. rules, in effect as such for a decade or more, taken directly from the current U.S. Conference of Catholic Bishops (USCCB) Fast and Abstinence page are:[45][9]
Ash Wednesday and Good Friday are obligatory days of fasting and abstinence for Catholics. In addition, Fridays during Lent are obligatory days of abstinence.
For members of the Latin Catholic Church, the norms on fasting are obligatory from age 18 until age 59. When fasting, a person is permitted to eat one full meal, as well as two smaller meals that together are not equal to a full meal. The norms concerning abstinence from meat are binding upon members of the Latin Catholic Church from age 14 onwards.
While the rules of abstinence generally only allow seafood, there are a few exceptions. In parts of South America, especially in Venezuela, capybara meat is popular during Lent and Holy Week; in response to a question posed by French settlers in Quebec in the 17th century, beaver was classified as an exception, as it classified the swimming creature with fish.[23][24][25] Similarly unusual classifications of “fish” for fur traders near Detroit have led to a local tradition of eating muskrat during Lent.[26][27] The Archbishop of New Orleans said that “alligator is considered in the fish family” in 2010.[28][29][30] The legal basis for the classification of capybara, beaver, and muskrat as fish probably rests with the Summa Theologica of Thomas Aquinas, which bases animal classification as much on habit as anatomy.[31]
The Filet-O-Fish was introduced by McDonald’s to accommodate Catholics who abstained from meat on Fridays.[16][1
https://en.wikipedia.org/wiki/Fasting_and_abstinence_in_the_Catholic_Church