Lent is the most solemn of all fasting times in the Church. From Apostolic times until 1741, meat was never allowed in Lent. Even after it was permitted at some meals, Fridays and Saturdays remained mandatory days of complete abstinence in Lent into the early 1900s. Friday in particular, the most solemn of all days on account of our Lord's death on the Cross on Friday, was a mandatory day of abstinence all year round (and it still is!) Up until the 1917 Code of Canon Law, meat was not even allowed on Holy Days of Obligation which fell on a Friday outside of Lent. The Friday fast, like Sunday Mass, is integral to Catholic life. This post is based heavily on the research which is incorporated in "The Definitive Guide to Catholic Fasting and Abstinence." Read the full book for much more information relevant to this topic.
Definition of Fasting vs. Abstinence
Fasting refers to how much food we eat and, historically, when we eat it. It means taking only one meal during a calendar day. The meal should be an average-sized meal as overeating at the one meal is against the spirit of the fast. Fasting generally means that the meal is to be taken later in the day. Along with the one meal, up to two snacks (technically called either a collation or frustulum) are permitted. These are optional, not required. Added up together, they may not equal the size of the one meal. No other snacking throughout the day is permitted.
Abstinence in this context refers to not eating meat. Meat refers to the fleshmeat of mammals or fowl. Beef, poultry, lamb, etc are all forbidden on days of abstinence. Abstinence does not currently prohibit animal byproducts like dairy (e.g. cheese, butter, milk) or eggs, but in times past they were prohibited. Fish is currently permitted along with shellfish and other cold-blooded animals like alligators. In times past, days of fast were always days of abstinence as well; however, not all days of abstinence were days of mandatory fasting.
The Church's Law in 1917
The days of obligatory fasting as listed in the 1917 Code of Canon Law were the forty days of Lent (including Ash Wednesday, Good Friday, and Holy Saturday until noon); the Ember Days; and the Vigils of Pentecost, the Assumption of the Blessed Virgin Mary, All Saints, and Christmas. Partial abstinence, the eating of meat only at the principal meal, was obligatory on all weeks of Lent (Monday through Thursday). And of course, complete abstinence was required on all Fridays, including Fridays of Lent, except when a holy day of obligation fell on a Friday outside of Lent. Saturdays in Lent were likewise days of complete abstinence.
The Modern Church Law
The National Conference of Catholic Bishops issued a statement on November 18, 1966. Abstinence was kept obligatory on all Fridays of Lent, except Solemnities (i.e. First Class Feasts), on Ash Wednesday, and on Good Friday. Abstinence on all Fridays throughout the year was "especially recommended," and the faithful who did choose to eat meat were directed to perform an alternative penance on those Fridays outside of Lent, even though the US Bishops removed the long-establish precept of requiring Friday penance. The document stated in part: "Even though we hereby terminate the traditional law of abstinence binding under pain of sin, as the sole prescribed means of observing Friday, we ... hope that the Catholic community will ordinarily continue to abstain from meat by free choice as formerly we did in obedience to church law." And finally, fasting on all weekdays of Lent was "strongly recommended" but not made obligatory under penalty of sin.
The 1983 Code of Canon Law largely took Paul VI's apostolic constitution aside from the modification of the age at which fasting binds. Per the 1983 Code of Canon Law, the age of fast was changed to begin at 18 - previously it was 21 - and to still conclude at midnight when an individual completes his 59th birthday. Friday penance is required per these laws on all Fridays of the year except on Solemnities, a dramatic change from the previous exception being only on Holy Days of Obligation.
Per the 1983 Code of Canon Law, fasting and complete abstinence per these rules are required only on Ash Wednesday and Good Friday. The notion of "partial abstinence," introduced under Pope Benedict XIV in 1741, was also removed. By this point, the days of obligatory fast had been reduced to merely two days. And most Catholics only abstained from meat on the 7 Fridays in Lent.
"The observance of Lent is the very badge of the Christian warfare. By it we prove ourselves not to be enemies of the cross of Christ. By it we avert the scourges of divine justice. By it we gain strength against the princes of darkness, for it shields us with heavenly help. Should mankind grow remiss in their observance of Lent, it would be a detriment to God's glory, a disgrace to the Catholic religion, and a danger to Christian souls. Neither can it be doubted that such negligence would become the source of misery to the world, of public calamity, and of private woe."
But it will be asked: “Are there, then, no lawful dispensations?” We answer that there are; and that they are more needed now than in former ages, owing to the general weakness of our constitutions. Still, there is great danger of our deceiving ourselves. If we have strength to go through great fatigues when our own self-love is gratified by them, how is it we are too weak to observe abstinence? If a slight inconvenience deters us from doing this penance, how shall we ever make expiation for our sins? For expiation is essential painful to nature.