The words disciple and discipline are linked, they are variations of the same root. The disciplines of the Master are practiced by disciples. The inward disciplines, those things we do individually, are summarized well by Richard Foster in his classic book, which is an essential guide for anyone who wants to draw closer to Christ, but needs a how to guide, The Celebration of Discipline: The Path to Spiritual Growth. These disciplines are:
Fridays are penitential days, days of abstinence, during which we, as Catholics, abstain from the meat of flesh animals, that is red meat, fowl, and other warm-blooded animals. Hence, the moniker mackrel snappers. But, it is also a traditional day of fasting. Friday as a penitential day was not done away with by Vatican II. It is no longer a matter of legal (i.e., canonical) obligation. The legal requirement was done away with so people could do works of penance and charity on Friday voluntarily and more out of intention than it being a rule. So, with Christmas drawing to a close, it is time to think about deepening our relationship Jesus, the babe of Bethlehem.
I refer you to one of my many posts on Friday as a day of penance, No Frills Friday