CCC 2043…The fifth precept (“You shall observe the prescribed days of fasting and abstinence.") ensures the times of ascesis and penance which prepare us for the liturgical feasts; they help us acquire mastery over our instincts and freedom of heart.
From your link:
Friday penance, therefore, is not a matter of mere counsel, but of actual precept. In plain language, a Catholic commits sin if he or she allows a Friday to pass without an act of penance. In Pope Paul’s Constitution on the subject entitled Poenitemini (which is the imperative of the verb “Repent”), after the Holy Father enumerates the days of penance, he states, “The substantial observance of these days binds gravely.” It may be recalled that there were some questions among commentators after the constitution was issued as to how this phrase should be interpreted. Did it refer to the days taken singly, so that on each Friday there was a grave obligation to penance with due allowance for slightness of matter, or did substantial observance mean that the days were to be taken collectively and only then was the obligation binding under mortal sin? The question has been authentically answered by the Sacred Congregation for Divine Worship, that it does not necessarily refer to each day, but that a person would sin seriously who omitted a part of the Friday penitential observance prescribed as a whole, if the part omitted were notable with regard to either quantity or quality and there was no excusing cause.
In the light of this teaching of the Church, when would a person be guilty of serious sin by not observing Fridays as days of penance? A practical answer is when he or she had failed to observe a notable number of Fridays, without proportionally grave reason.
I was not too concerned whether it is a mortal sin or not. The Church gives us the means to know whether our sin is mortal or not. Your confessor will also be a guide.
CCC 1857 For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”
CCC 1862 One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.
CCC 1863 Venial sin weakens charity; it manifests a disordered affection for created goods; it impedes the soul’s progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment. Deliberate and unrepented venial sin disposes us little by little to commit mortal sin. However venial sin does not set us in direct opposition to the will and friendship of God; it does not break the covenant with God. With God’s grace it is humanly reparable. “Venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness.”
The purpose of emailing the USCCB was to correct posts like these:
I did not include the name, or any other information, of the responder, as this is a private email. I have replied to the response and asked for permission. If, and when, I receive this, I will post it.