Contemporary Vatican legislation, which is followed by Catholics of the Latin Rite (who comprise most Catholics) is rooted in the 1966 Apostolic Constitution of Pope Paul VI, Paenitemini, and codified in the 1983 Code of Canon Law (in Canons 1249–1253). According to Paenitemini and the 1983 Code of Canon Law, on Ash Wednesday and Good Friday, both abstinence and fasting are required of Catholics who are not exempted for various reasons. All Fridays of the year are days of penance. All persons who are aged fourteen or older are bound by the law of abstinence on all Fridays that are not Solemnities, while the law of fasting binds all Catholics who are aged between eighteen and sixty on Ash Wednesday and Good Friday. Nevertheless, both Paenitemini and the 1983 Code of Canon Law permitted the Episcopal Conferences to propose adjustments of the laws on fasting and abstinence for their home territories, and most have done so. For example, in some countries, the Bishops' Conferences have obtained from Rome the substitution of pious or charitable acts for abstinence from meat on all Fridays of the year (including Fridays of Lent) except Good Friday. Others continue to abstain from eating meat on Lenten Fridays, but not on Fridays outside of Lent. Still others voluntarily abstain from meat on Fridays throughout the year. Article 110 of Sacrosanctum Concilium designates Good Friday and Holy Saturday as the Paschal fast; however, fasting may be less stringent on Holy Saturday than on Good Friday.
Members of the Eastern Catholic Churches are obliged to follow the discipline of their own particular church. While some Eastern Catholics try to follow the stricter rules of their Orthodox counterparts, the actual canonical obligations of Eastern Catholics to fast and abstain are usually much more lenient than those of the Orthodox.
Some parishes of the Personal Ordinariates for former Anglicans reconciled to the Catholic Church have either continued or re-established the discipline of Friday abstinence as conceived in the Book of Common Prayer. Early Prayer Books set out rules that were in line with the Sarum Rite of the time, where most days prior to Solemnities and Feasts were delegated as "days of abstinence" along with the Rogation Days. The eating of fish on these days is generally ruled out within the English Patrimony of the Personal Ordinariate of the Chair of Saint Peter so the faithful are effectively vegetarian for the day.
Rules relating to fasting pertain to the quantity of food allowed on days of fasting, while those regulating abstinence refer to the quality or type of food. The Christian tradition of fasts and abstinence developed from Old Testament practices, and were an integral part of the early church community. Louis Duchesne observed that Monday and Thursday were days of fasting among pious Jews. Early Christians practiced regular weekly fasts on Wednesdays and Fridays.
The habit of fasting before Easter developed gradually, and with considerable diversity of practice regarding duration. As late as the latter part of the second century there were differing opinions not only regarding the manner of the paschal fast, but also the proper time for keeping Easter. In 331 St. Athanasius enjoined upon his flock a period of forty days of fasting preliminary to, but not inclusive of, the stricter fast of Holy Week, and in 339, after having traveled to Rome and over the greater part of Europe, wrote in the strongest terms to urge this observance upon the people of Alexandria as one that was universally practiced, "to the end that while all the world is fasting, we who are in Egypt should not become a laughing-stock as the only people who do not fast but take our pleasure in those days".
In the time of Gregory the Great (590–604) there were apparently at Rome six weeks of six days each, making thirty-six fast days in all, which St. Gregory, who is followed therein by many medieval writers, describes as the spiritual tithing of the year, thirty-six days being approximately the tenth part of three hundred and sixty-five. At a later date the wish to realize the exact number of forty days led to the practice of beginning Lent on Ash Wednesday.