Saturdays (Sundays to a lesser degree) are crowded because locals and quick-weekend trip people fill it up.
Full-scale vacationers tend to fill the MK on Mondays. The thought process is, "We came to Disney, and MK is the main thing we want to see, so we're going to do it first."
I'd say Tuesday, Wednesday and Thursday are the best days for MK. Having said that, check the hours; the parks may close earlier mid-week. And check for fireworks and parades -- they may not be offered mid-week.