The Second Precept of the Church, To fast on the days prescribed, bids us keep the fast:
Fasting consists in taking only one meal a day, and in abstaining from forbidden foods. Through the condescension of the Church, however, on fast days a collation is allowed at the evening meal. The object of fasting is to dispose us better to pray, to do penance for past sins, and to prevent the commission of fresh sins.
All Christians are bound to fast, if they are 21 years of age, and have not been dispensed, or are not excused by some lawful impediment; but those who are not bound to fast are not exempt from mortification, because all are bound to penance.
Lent was instituted in order to imitate the some extent the strict fast of 40 days which Jesus Christ went through in the wilderness, and to prepare us by penance to keep Easter in a holy manner. The Advent fast is meant to prepare us to keep the Feast of Christmas in the same way; and the Ember fasts are intended to give a consecration to every season of the year by some days of penance, to ask God to guard the fruits of the earth and to thank Him for those which He has already given, and to beg Him to give His Church good ministers, as their ordination takes place on the Saturdays of the four seasons. The fasts on Vigils are prescribed to prepare us for the holy celebration of the principal Feasts.
On Friday and on Saturday, if not dispensed, we are forbidden to eat meat except in case of necessity. This is to enable us to do penance every week, especially on Friday in honor of the Passion, and on Saturday in memory of the burial of Jesus Christ, and in honor of our Blessed Lady.
483. What is commanded by the Second Precept of the Church, To fast on the days prescribed?
By the words, To fast on the days prescribed, the Second Precept of the Church commands us to observe the fast: 1. during Lent; 2. on certain days of Advent, where this is prescribed; 3. on Ember Days; 4. on certain Vigils.
484. In what does fasting consist?
Fasting consists in taking but one meal a day and in abstaining from prohibited kinds of food.
485. On fast days may a collation be taken in the evening?
Through the concession of the Church, a collation may be taken in the evening on fast days.
486. What is the good of fasting?
Fasting serves to dispose us better to prayer; to do penance for past sins; and to preserve us from sinning again.
487. Who are bound to fast?
Every Christian over twenty-one years of age who is not dispensed, or excused for some good reason, is bound to fast.
488. Are those who are not bound to fast exempt from all mortification?
Those who are not bound by the obligation of fasting are not exempt from all mortification, because all are bound to do penance.
489. For what purpose has Lent been instituted?
Lent has been instituted to imitate in some way the rigorous fast of forty days undergone by Jesus Christ in the desert, and to prepare us by penitential exercises to celebrate the Feast of Easter devoutly.
490. Why has the Advent fast been instituted?
The Advent fast has been instituted to prepare us to celebrate devoutly the Feast of our Lord's Nativity.
491. Why has the fast of Ember Days been instituted?
The fast of Ember Days has been instituted: 1. to consecrate each of the four seasons of the year by some days' penance; 2. to beg of God the preservation of the fruits of the earth; 3. to thank Him for those already given us; 4. to beseech Him to give good priests to His Church, the usual days for ordaining priests being the Ember Saturdays.
492. Why has fasting on Vigils been instituted?
Fasting on Vigils has been instituted to prepare us to celebrate the principal Feasts devoutly.
493. What is forbidden on Fridays, and also on Saturdays where not dispensed?
On Fridays, and also on Saturdays where not dispensed, it is forbidden to eat meat, except in case of necessity.
494. Why does the Church wish us to abstain from eating meat on these days?
The Church wishes us to abstain from meat on these days in order that we may do penance each week, and especially on Friday, in honor of the Passion, and on Saturdays in memory of the burial of Jesus Christ, and in honor of the Blessed Virgin.
NOTE: From the Code of Canon Law of 1917, with the modifications approved in 1949, (complete) abstinence only is to be observed on all the Fridays throughout the year. Fast and (complete) abstinence is to be observed on Ash Wednesday, the Fridays of Lent, Ember Fridays, the Vigils of the Assumption and Christmas, and Holy Saturday. Fast only (and partial abstinence) is to be observed on: all weekdays of Lent, Ember Wednesdays and Saturdays, and the Vigils of Pentecost and All Saints.
On the days of fast, only one full meal is allowed. Two other meatless snacks, sufficient to maintain strength may be taken according to each one's needs, but together they should not equal another full meal. Meat may be taken at the principal meal on a day of fast except on the days of complete abstinence. Eating between meals is forbidden, but liquids including milk and fruit juices are allowed.
On feasts of obligation, except in Lent, there is neither fast nor abstinence.
The laws of abstinence binds all who have completed their seventh year of age; the law of fasting binds all persons from the completion of their twenty first year until the beginning of their sixtieth. (The completion of the seventh year means the day after the seventh birthday).
According to the 1983 Code of Canon Law, "All Fridays through the year and the time of Lent are penitential days and times throughout the universal Church. Abstinence from eating meat . . . is to be observed on Fridays throughout the year unless they are solemnities; abstinence and fast are to be observed on Ash Wednesday and (Good) Friday. All adults who have completed their fourteenth year are bound by the law of abstinence; all adults (from age 18) are bound by the law of fast up to the beginning of their sixtieth year." The Episcopal Conference can modify these general rules.
In the United States, "Catholics are obliged to abstain from the eating of meat on Ash Wednesday and all Fridays during the season of Lent. They are also obliged to fast on Ash Wednesday and on Good Friday. Self-imposed observances of fasting on all weekdays of Lent is strongly recommended. Abstinence from flesh meat on all Fridays of the year is especially recommended to individuals and to the Catholic community as a whole."
In Australia, "Abstinence from meat, and fasting, are to be observed on Ash Wednesday and Good Friday. On all other Fridays of the year the law of the common practice of penance is fulfilled by performing any of the following: (a) prayer, as for example Mass attendance, family prayer, . . . (b) self-denial, e.g. not eating meat, not eating sweets or dessert, . . (c) helping others, e.g. special attention to someone who is poor, sick, elderly, . . ."
Since a clarity and precision in a law helps for its fulfillment, the faithful are strongly recommended to follow the traditional rules, though only the recent regulations oblige under pain of mortal sin.