Glory to Jesus Christ!
Two points I would like to add to the discussion, first of all the early Christians actually originally fasted on Wednesdays and Fridays and I am surprised that this never comes up in discussion.
Our Lord was betrayed on Wedneday and brutalized on Friday, He died to this life for all of us that day and rose again on the eigth day. We could always keep this in mind with a weekly pattern of fast-fast-feast, just as the early Catholics did and many Orthodox still do.
In earlier times people had a life of Faith, and did not question whether a sin was mortal or not, or how the sin (or lack of) is specifically explained. But in the West it has been so long since the average person lived like this that almost no one realizes that fasting would have been on Wednesday too.
Our whole society is so lax about matters of Faith and practice that most Catholics are surprised to learn that we still have a fasting rule at all.
Metal1633 was correct, as usual, but I cannot provide references right now.
Which brings me to my second point. I understood that we are to do some other form of penance if we do not fast, as our choice. I was surprised to learn of the opinion that the bishops should have picked a form of penance for us to do, I never heard that before. We should be mature enough and enlightened enough to do this without so much direction.
I would recommend some scripture study on Friday, or volunteering help to some worthy endeavor. Possibly write a big check to a charity or do errands with the Society of St Vincent De Paul.
Today, even staying off of the internet for 24 hours would seem like a huge sacrifice to some people! Or staying off the telephone!
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But I am not suggesting that.