Dear Don,

Thanks for the question.

The verse in question can only be understood in the larger context of Paul’s instructions in chapters 13 and 14 and really throughout the whole letter.  Since sharp divisions had arisen between Christians who adhered to Jewish customs and laws versus those who did not, Paul was trying to remind them to preserve unity and charity despite differences.  He was in effect making these practices a sense of personal piety which shouldn't be imposed on others, and also he admonished them not to judge those who didn't have the same types of devotions.  Paul had a respect for true diversity, but nonetheless was ardent about the glue that held all Christians together.

So, “keep it to yourself” or “between you and God” is like saying don’t expect everyone to worship in some cookie cutter carbon copy of yourself; we are each different.  Some folks fast at certain times because their personal sense of piety compels them to, and we should let them.  

If devotion isn't hindering faith, “let me do me and you do you” in today's parlance.  Just be charitable toward all and don’t judge your brother's personal piety (especially if he comes from a different culture and is not doing something heretical — there will be diversity in this regard.)
Peace,