To anyone who is still having confusion about this topic: please read the entire quote posted by PaulfromIowa above! It gives you all the information you need!
I also want to say that Friday penance is required for the reason that penance on earth wipes away time from Purgatory, so it is very good and healthy for us Catholics to do!
Also, it is important to remember that penance is not required, and actually discouraged, when a solemnity falls on a Friday. The Fridays within the Octaves of Christmas and Easter are also treated as solemnities in terms of celebration. Therefore, penance is also discouraged from those days.
Finally, it should be known that, although honestly forgetting to do penance on a Friday is not sinful, doing so intentionally and out of spite is definitely a sin, because you are openly and unfortunately defying God.
Please, just do your penances, it does not take such a huge amount of effort to sacrifice something for Our Lord, Who sacrificed His Life for us.
May God bless you all!