After reading another thread I have a bit of confusion. Ok so I know we are asked to either abstain from meat or perform another penance of our choice on all non-Lenten Fridays throughout the year. But I was under the Impression that this was voluntary and no longer binding under pain of mortal sin. So if on non-Lenten Fridays you did absolutely no penance then, while not ideal, it was not a mortal sin. Could someone kindly clear this up for me please?
Yes, every Friday is a day of penance. The penance is obligatory, not voluntary.
Can. 1251 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.
See also canon 1253
Can. 1253 The conference of bishops can determine more precisely the observance of fast and abstinence as well as substitute other forms of penance, especially works of charity and exercises of piety, in whole or in part, for abstinence and fast.
The form that the penance might take is voluntary. The universal norm is to abstain from meat, however (in the U.S.) an individual is free to substitute his own choice of penance in its place, this without any need to consult a pastor or confessor or anyone else.
The U.S. bishops did not do away with the obligation of Friday penance. That is beyond their authority. They can substitute, they cannot eliminate.
Failing to do some form of penance on Friday is not a mortal sin; if anything it would be venial. It would be a sin of omission, but one that would depend on circumstances.