Why does the Church make Fridays of Lent days of abstinence? The Church makes Fridays of Lent days of abstinence in order that we may do penance for our sins, and also in order that we may prepare ourselves more worthily for Good Friday, when we commemorate the death of Jesus Christ.
How can we know the days appointed for fast or abstinence? We can know the days appointed for fast or abstinence from the instructions of our bishops and priests.
More on that -> CCC 1438