<<<<< Remember there are three parts to the repentence (in this order): contrition, confession, and satisfaction. Fasting before confession would be fine as a sign of contrition–but if you were not in the state of grace it would not have satisfactory merit.
After confession and you have been restored to grace, you should make your satisfaction. The penances of a few prayers we are generally given to begin the satisfaction (and sometimes complete it), but we can still do additional penances or obtain indulgences to make the complete satisfaction (or even more than is necessary to contribute to the Church’s “treasury of merit.”)
That would be the time to fast, while repentence is still fresh in your mind, before feasting, IMO. >>>>>
Mal: Well said and to augment what you said - even when a Sin is forgiven in the Confessional and the penance performed - that only serves to make satisfaction to God for the Guilt of the Sin. Not the satisfaction from temporal punishment. We willl repay to GOD every last farthing either in this life or in the next and depending on the severity of the Sin - some time in purgatory may be in order unles we make satisfaction in this life through prayers , penance, suffering , alms deeds , charity , fastings etc. Feasting doesnt help in this area.
pax