338. The second commandment of the Church is to fast and abstain on the days appointed. It appears to be of Apostolic origin; but it is ever adapted by the authority of the Church to the changing circumstances of times and places. Since it regards important matters, it carries with it a grievous obligation; still a slight transgression of the law constitutes only a venial sin.
The precept of fasting obliges the faithful who have completed the twenty-first, but not the sixtieth year of their lives, to take only one full meal a day, about noon or after, on all the week-days in Lent, on the Ember-days, and on the vigils of Christmas, Pentecost, the Assumption of the Blessed Virgin, and of All-Saints-day also, except in some dioceses of this country, on the Fridays in Advent. Besides the one full meal two ounces of solid food are allowed in the morning, and a collation, or light supper, at night, at which eight ounces of solid food may be taken. More is allowed if more is needed by any one in order to continue the fast for successive days without injury to health. Drink does not break the fast; but milk is considered rather food than drink.
The Church excuses from the law of fasting a) All those employed in hard and prolonged bodily labor b) The sick and infirm generally c) Pregnant and nursing women; d) The very poor, who cannot usually procure very nourishing food. In doubt as to the sufficiency of the excuse, the proper course is to consult one's pastor or confessor; these, besides being safe interpreters of the law, can in certain cases grant dispensations from its obligations.
339. The law of abstinence, as now in force in the United States, forbids no other food than flesh-meat; this it forbids in all its forms, including meat soups and sauces, except however lard used as a substitute for butter. This law obliges all the faithful who have the use of reason to abstain from flesh-meat on all Fridays, except on Christmas when it falls on that day; also on all fast-days (n. 338) except the Mondays, Tuesdays, and Thursdays in Lent, and except all the Saturdays of Lent but the second and the last. Those obliged to fast may not eat meat more than once on the days on which it is allowed ; and none of the faithful, except the sick, may eat flesh-meat and fish at the same meal on a fast-day or a Sunday in Lent.
It belongs to the Bishops to make such special regulations for the observance of the general laws of fast and abstinence, as they deem proper for the faithful of their diocese.
340. The practices of fasting and of abstaining from special kinds of food and drink, and other species of mortification, are highly recommended to all the faithful without exception, provided they be restrained within the proper bounds of Christian prudence. For they are taught us by the example of Christ and the Saints, and are inculcated in numberless passages of Holy Scripture. They are often necessary to weaken concupiscence and to obtain the grace of resisting temptations. St. Paul teaches this when he writes: "I chastise my body and bring it into subjection; lest perhaps, when I have preached to others, I myself should become a castaway" (1 Cor. IX, 27). These acts of penance are among the most efficient means to obtain pardon of sin, and any favors we may desire from the liberality of God; as we see exemplified in the pardon which He granted to the Ninivites (Jon. III).