As early as the 2nd century, the Didache notes the practice of abstaining from meat on all Fridays of the year as a penitential observance recalling the crucifixion. There is no corresponding insistence on eating fish. (Some members of my family consider seafood a form of poison, so not everyone shares your enthusiasm for it.) It was Pope Nicholas I (9th c.) who made this practice binding under pain of mortal sin—and not because his family owned a fish market, as is sometimes suggested. Pope Innocent III (12th c.) made an exception for when Christmas falls on a Friday.

Thomas Aquinas considered meat, milk, and eggs all foods that incite desire. Fasting and abstinence were meant to bridle "the concupiscences of the flesh, which regard pleasures of touch in connection with food and sex.” Vegans might find incidental common cause with this doctor of the church.

It wasn't until 1966 that Pope Paul VI advised local church officials to modify the abstinence rule as they saw fit. That same year, U.S. Bishops issued the Pastoral Statement On Penance And Abstinence allowing a substitution of some other form of penance in place of abstinence on all Fridays except for those that occur in Lent. However, persons in good health between the ages of 14 and 59 must abstain from meat (and items made with meat) on Ash Wednesday, Good Friday, and lenten Fridays.

Some bishops or pastors make exceptions for St. Patrick's and/or St. Joseph's Day when they fall on lenten Fridays. In 2012, the U.S. bishops reconsidered reinstituting abstinence for all Fridays of the year, but preferred to make it optional to abstain on Friday for the intentions of life, marriage, and religious liberty. In 2010 the bishop of New Orleans reclassified alligator as a non-meat item on the menu.

The practice of abstinence from meat is intended as a penitential practice. Obviously, if you're wild about fish, that may not be the best substitution with which to observe the sacrifice. While fish, lobster and other shellfish are not categorized as meat and can be consumed without violating abstinence, indulging in a seafood buffet isn't in the spirit of a penitential act. 

Scripture: Pss. 69:11; 109:24; Isa 58:3-12; Dan 9:3; Joel 2:12-17; Neh 1:4; 9:1;             Tobit 12:8; Judith 4:13; Esther 4:3; 9:31; Matt 6:16-18; 9:14-15; Lk 2:37; Acts 13:2-3; 14:23

Books: The Origins of Feasts, Fasts, and Seasons in Early Christianity - Paul Bradshaw and Maxwell Johnson (Collegeville, MN: Liturgical Press, 2011)

The Spirituality of Fasting: Rediscovery of a Christian Practice - Charles M.Murphy (Notre Dame, IN: Ave Maria Press, 2010)

The term Jubilee was adopted by Catholics in 1300 when the first Holy Year was inaugurated. It too is a year of liberation, though of a spiritual nature. A Holy Year is a time when the pope offers special spiritual privileges, or indulgences, for those who make a pilgrimage to Rome. Other religious activities may be assigned similar benefits during the Holy Year for those unable to travel. The Holy Year was originally intended as a centennial observance. But it came to be observed every 25 years, and additional celebrations between those intervals can be added when deemed helpful.

The Holy Year generally begins on Christmas Eve with the opening of an official Holy Door at St. Peter's Basilica in Rome which is normally kept bricked up in other years. Three other Roman Basilicas—St. Paul's Outside the Walls, St. John Lateran, and St. Mary Major—also have doors opened to encourage pilgrimage. Bishops around the world are invited to designate similar doors within their jurisdictions through which pilgrims can be welcomed to celebrate the spiritual benefits of the event. Even religious educators are being asked to ritually open a door with their students in order to instruct them about the purpose of the Holy Year.

The Holy Year of Mercy called for by Pope Francis began this year on the Solemnity of the Immaculate Conception, December 8, 2015. This Solemnity recalls the occasion when God opened an avenue of mercy for the human race through the incarnation of Jesus. "Jesus Christ is the face of the Father's mercy," Pope Francis declares. By symbolically opening a door, we spread the good news that anyone who seeks the love of God in Jesus will find consolation, pardon, and hope. All of us should eagerly seek the occasion to walk through that door. We should also extend the invitation to anyone in need of the assurance that God's mercy is open to them.

Scripture: Lev 25:8-55; Isa 5:8-10; Ezek 46:17

Books: Crossing the Threshold of Mercy ed. Mark-David Janus (Mahwah, NJ:Paulist Press, 2015)

Beautiful Mercy: Experiencing God's Unconditional Love So We Can Share It With Others Pope Francis, Matthew Kelly, et. al. (Beacon Publishing, 2015)