Is it reprehensible or undesirable to single out Friday for visiting graves?

The act of specifically visiting graves on Friday cannot be separated from two scenarios: Either one believes there is a special virtue in doing so beyond visiting on other days, and this is an innovation (bid'ah) that must be avoided. Or it is because Friday is your day off, in which case there is no harm in it, although it is preferable to avoid doing it consistently so that people do not assume it is a Sunnah.