We are in the Octave of Christmas, that stoppage of liturgical time when we can rest in place and contemplate the mystery of the Incarnation from various angles.
During the Octave of Easter, we are not obliged to do penance on Easter Friday. What about Friday in the Christmas Octave?
Days (other than Sunday) within the Octave of Christmas are not “heavy enough” (as a “solemnity” would be) to “outweigh” the Friday obligation to do some sort of penance as determined by the conferences of bishops.
In the 1962 Missale Romanum they are “II class”, which corresponds to the newer, non-traditional calendar’s “feast”. In the 2001 Missale Romanum they are categorized as second class, as “feasts”. If, however you are today at a parish named “St. John the Evangelist”, it is your parish’s patronal feast. You might argue that today you don’t have to stick to doing the penance we are obliged to do on Fridays.
Bottom line, the Octave of Christmas does not have the “weight” of the Octave of Easter. Therefore, we have to do some sort of penance today.
Let’s also pay attention to can. 1245. You can ask your parish priest to dispense you or commute your act of penance.
Can. 1245 Without prejudice to the right of diocesan bishops mentioned in can. 87, for a just cause and according to the prescripts of the diocesan bishop, a pastor [parish priest] can grant in individual cases a dispensation from the obligation of observing a feast day or a day of penance or can grant a commutation of the obligation into other pious works. A superior of a religious institute or society of apostolic life, if they are clerical and of pontifical right, can also do this in regard to his own subjects and others living in the house day and night.
Moreover, you can substitute another form of penance for abstaining from meat. Make it penitential, however. Abstinence from meat has good reasoning behind it. For some, however, there abstinence from other things can be of greater spiritual effect.