Response: As part of our response to Christ’s exhortation to repent, the Church urges us to live penitentially every day. While Catholics are called to do penance primarily in daily life, the Church has established special days of penance—all Fridays and the season of Lent—on which Catholics are called to pray, fast, and perform works of charity. The laws of fast and abstinence bind Catholics of certain ages on certain days of penance.
Discussion: Repentance is at the heart of the Gospel. After the beginning of His public life, Jesus “came into Galilee, preaching the gospel of God, and saying, ‘The time is fulfilled, and the kingdom of God is at hand; repent, and believe in the Gospel’" (Mk. 1:14-15). Later, when He began to send out the Twelve two by two, “they went out and preached that men should repent" (Mk. 6:12). On Pentecost, St. Peter’s hearers “were cut to the heart, and said to Peter and the rest of the apostles, ‘Brethren, what shall we do?’ And Peter said to them, ‘Repent, and be baptized every one of you" (Acts 2:37-38).
The resounding message of the Gospel—"Repent!"—is primarily an invitation to “the conversion of the heart, interior conversion. Without this, [exterior] penances remain sterile and false; however, interior conversion urges expression in visible signs, gestures and works of penance." A “work of the grace of God who makes our hearts return to him," interior repentance is “a radical reorientation of our whole life, a return, a conversion to God with all our heart, an end of sin, a turning away from evil, with repugnance toward the evil actions we have committed. At the same time it entails the desire and resolution to change one’s life, with hope in God’s mercy and trust in the help of his grace."
Since 1966, Pope Paul VI’s apostolic constitution Paenitemini (“Repent") has governed the Church’s penitential discipline. In guiding our response to Christ’s exhortation to repent, the document urges us to live penitentially every day, for as the Catechism of the Catholic Church would teach decades later, “taking up one’s cross each day and following Jesus is the surest way of penance." Thus the Church “insists first of all that the virtue of penitence be exercised in persevering faithfulness to the duties of one’s state in life, in the acceptance of the difficulties arising from one’s work and from human coexistence in a patient bearing of the trials of earthly life and of the utter insecurity which pervades it."
We ought, then, to look at our state in life and the difficulties inherent in the human condition as the means by which God invites us to daily repentance. We respond to this invitation by fulfilling our duties faithfully and by accepting such difficulties patiently.
In addition to these day-to-day trials, God, in His loving providence, sends more specific, personal trials into His children’s lives. These personal sufferings are acts of discipline by which our loving Father leads us into deeper repentance and transforms us into truer disciples of Christ. “God is treating you as sons; for what son is there whom his father does not discipline? . . . He disciplines us for our good, that we share his holiness" (Heb. 12:7, 10). Our sufferings are not for our own benefit alone, for they can lead us to say with St. Paul, “Now I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ’s afflictions for the sake of his body, that is, the church" (Col. 1:24). Thus the Church teaches that “those members of the Church who are stricken by infirmities, illnesses, poverty or misfortunes, or who are persecuted for the love of justice, are invited to unite their sorrows to the suffering of Christ in such a way that they not only satisfy more thoroughly the precept of penitence but also obtain for the brethren a life of grace and for themselves that beatitude which is promised in the Gospel to those who suffer."
Our daily repentance, brought about by grace through the duties of our state of life, the difficulties inherent in the human condition, and the specific sufferings God sends us, finds its “source and nourishment in the Eucharist." External signs accompany our daily interior conversion, as the Catechism elaborates:
Conversion is accomplished in daily life by gestures of reconciliation, concern for the poor, the exercise and defense of justice and right, by the admission of faults to one’s brethren, fraternal correction, revision of life, examination of conscience, spiritual direction, acceptance of suffering, [and] endurance of persecution for the sake of righteousness.
Days of Penance: The Universal Discipline
Within this context of penitential discipleship, the Church invites the faithful to “respond to the divine precept of penitence by some voluntary act, apart from the renunciation imposed by the burdens of everyday life."
Holy Mother Church, although it has always observed in a special way abstinence from meat and fasting, nevertheless wants to indicate in the traditional triad of “prayer-fasting-charity"  the fundamental means of complying with the divine precepts of penitence. . . . Where economic well-being is greater, so much more will the witness of asceticism have to be given in order that the sons of the Church may not be involved in the spirit of the “world," and at the same time the witness of charity will have to be given to the brethren who suffer poverty and hunger beyond any barrier of nation or continent. . . . In order that all the faithful . . . may be united in a common celebration of penitence, the Apostolic See intends to establish certain penitential days and seasons chosen among those which in the course of the liturgical year are closer to the paschal mystery of Christ or might be required by the special needs of the ecclesial community.
These common penitential days and seasons, on which we are exhorted to pray, fast, and perform works of charity, are “all Fridays through the year and the time of Lent."
Particularly appropriate acts of prayer on Fridays and during the season of Lent include reading Sacred Scripture, praying the Liturgy of the Hours, praying the Our Father, making spiritual exercises (that is, a retreat or day of recollection), attending penitential liturgies (especially the Sacrament of Penance), and making pilgrimages.  Still, the Church does not require that any specific prayer be said on these days, for “every sincere act of worship or devotion revives the spirit of conversion and repentance within us and contributes to the forgiveness of our sins."
To assist the faithful in fasting during these times of penance, the Church has legislated the following:
Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity  should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday.
The law of abstinence binds those who have completed their fourteenth year. The law of fasting binds those who have attained their majority, until the beginning of their sixtieth year. Pastors of souls and parents are to ensure that even those who by reason of their age are not bound by the law of fasting and abstinence, are taught the true meaning of penance.
Bishop and “pastors also for just cause and in accordance with the prescriptions of the Ordinary may grant to individual faithful as well as individual families dispensation or commutation of abstinence and fast into other pious practices."
While the Church does not specify how we should engage in “fraternal sharing (charitable and missionary works)" during penitential times, the Catechism in its treatment of the Seventh Commandment teaches that
The works of mercy are charitable actions by which we come to the aid of our neighbor in his spiritual and bodily necessities. Instructing, advising, consoling, [and] comforting are spiritual works of mercy, as are forgiving and bearing wrongs patiently. The corporal works of mercy consist especially in feeding the hungry, sheltering the homeless, clothing the naked, visiting the sick and imprisoned, and burying the dead. Among all these, giving alms to the poor is one of the chief witnesses to fraternal charity: it is also a work of justice pleasing to God.
Within the context of daily penitential discipleship, then, the Church has established all Fridays and the season of Lent as times of penance in which prayer, fasting, and works of charity hold pride of place. According to the discipline of the universal Church, Catholics of certain ages must fast and abstain from meat on Ash Wednesday and Good Friday and abstain from meat on all Fridays that are not solemnities.
Days of Penance: The American Discipline
Pope Paul VI granted episcopal conferences wide latitude in implementing and modifying the universal discipline of penance: “It is up to the bishops—gathered in their episcopal conferences—to establish the norms which, in their pastoral solicitude and prudence, and with the direct knowledge they have of local conditions, they consider the most opportune and efficacious." Nine months after Pope Paul reformed the Church’s universal penitential discipline, the American bishops responded to his invitation to apply that discipline in the United States. In doing so, the American bishops made four modifications.
First, even though Advent is no longer a season of penance, the American bishops urged the Catholics of the United States to observe Advent as a penitential time of preparation for Christmas.
Second, the American bishops offered specific guidance on observing the weekdays of Lent on which there is no mandatory fast or abstinence:
For all other weekdays of Lent, we strongly recommend participation in daily Mass and a self-imposed observance of fasting. In the light of grave human needs which weigh on the Christian conscience in all seasons, we urge particularly during Lent, generosity to local, national, and world programs of sharing of all things needed to translate our duty to penance into a means of implementing the right of the poor to their part in our abundance. We also recommend spiritual studies, beginning with the Scriptures as well as the traditional Lenten devotions (sermons, Stations of the Cross, and the Rosary) and all the self-denial summed up in the Christian concept of “mortification."
Let us witness to our love and imitation of Christ, by special solicitude for the sick, the poor, the underprivileged, the imprisoned, the bed-ridden, the discouraged, the stranger, the lonely, and persons of other color, nationalities or background than our own.
Third, even though Pope Paul VI had abolished the penances associated with ember days and the vigils of feast days, the American bishops stated:
The liturgical renewal and the deeper appreciation of the joy of the holy days of the Christian year will, we hope, result in a renewed appreciation as to why our forefathers spoke of “a fast before a feast." We impose no fast before any feastday, but we suggest that the devout will find greater Christian joy in the feasts of the liturgical calendar if they freely bind themselves, for their own motives and in their own spirit of piety, to prepare for each Church festival by a day of particular self-denial, penitential prayer, and fasting.
Fourth, the American bishops modified the discipline of abstinence on Fridays outside of Lent. “Since the spirit of penance," they wrote, “primarily suggests that we discipline ourselves in that which we enjoy most, to many in our day abstinence from meat no longer implies penance, while renunciation of other things would be more penitential." With the intention of reinvigorating the penitential observance of Friday, the bishops
urged Catholics to be mindful on Fridays of their sins and the sins of mankind, which we are called to help expiate,
asked Catholics to make Friday “in each week something of what Lent is in the entire year, “that is, “a day of self-denial and mortification in prayerful remembrance of the passion of Jesus Christ" in preparation for Sunday, the weekly Easter,
and abolished the mandatory nature of Friday abstinence outside of Lent but instead allowed Catholics to choose their own act of “voluntary self-denial and personal penance," among which abstinence stil holds pride of place.
For those Catholics who choose a penance besides abstinence from meat, the bishops recommended temperance in the use of stimulants and alcoholic beverages. They added:
It would bring great glory to God and good to souls if Fridays found our people doing volunteer work in hospitals, visiting the sick, serving the needs of the aged and the lonely, instructing the young in the faith, participating as Christians in community affairs, and meeting our obligations to our families, our friends, our neighbors, and our community, including our parishes, with a special zeal born of the desire to add the merit of penance to the other virtues exercised in good works born of living faith.
Since 1966, the American bishops have repeated the call to observe Friday as a day of penance. In a 1983 pastoral letter, they wrote:
As a tangible sign of our need and desire to do penance we, for the cause of peace, commit ourselves to fast and abstinence on each Friday of the year. We call upon our people voluntarily to do penance on Friday by eating less food and by abstaining from meat. This return to a traditional practice of penance, once well observed in the U.S. Church, should be accompanied by works of charity and service toward our neighbors. Every Friday should be a day significantly devoted to prayer, penance, and almsgiving for peace.