Mark 2:18-22 fasting is like pouring new wine into old wine skins or sewing new cloth into old garment / old garments are already shrunk, when old garments and cleaned and dried with new piece sewn in new piece with shrink and make a bigger tear, old wine skins have already been stretched, when new wine ferments it expands and if it is poured into a old skins, they don't stretch and will burst.
This is to Mary and Martha, where Martha is busy preparing the home and serving and Mary is sitting at Jesus' feet (so busy trying to righteous and do something righteous and when what we can do best is spend time with in Jesus' presence - Luke 10:38-42)
Fasting is no longer needed to atone for sins, if you try to take pieces of the law and sew them into new Christianity it will not work, similarly if you try to take new era of grace and try to squeeze it into the law it will not work, choose the better thing Grace given to us by Christ through our faith

It is just as important to prepare for the fast as it is to plan for breaking your fast (must go into and come out of the fast the right way)