Why were holydays instituted by the Church? Holydays were instituted by the Church to remind us of the mysteries of our religion and of the important events in the lives of Christ and of His Blessed Mother, and to recall to us the virtues and the rewards of the saints.
What is a fast day? A fast day is a day on which only one full meal is allowed, but in the morning and evening some food may be taken, the quantity and quality of which are determined by approved local custom.
Who are obliged to observe the fast days of the Church? All baptized persons between the ages of twenty-one and fifty-nine are obliged to observe the fast days of the Church, unless they are excused or dispensed.
More on that -> CCC 1430