The latest revision of the laws of fast and abstinence was made by Pope Paul VI in 1966 in Paenitemini. In that apostolic constitution he left certain things to the judgment of the national conferences of bishops.
The U.S. bishops determined that Catholics in the United States should fast and abstain on Ash Wednesday and Good Friday and abstain from meat on all Fridays during Lent.
The U.S. bishops recommend voluntary fasting throughout Lent and voluntary abstinence on all Fridays of the year.
In Paenitemini, Paul VI reminded us of the need we all have to do penance. And penance can take many forms: acts of charity like visiting the sick or people in jail, tutoring students, almsgiving, doing without candy, liquor, TV, and so on.
Finally, you can never commit a sin without knowing it. One of the conditions for sin is knowledge of the evil or disobedience involved.