Our experience has always been that Sundays have been one of the best days to go. But let me say up front that we are not AP holders so others may have more experience with this. Last year, the Sunday we were there was by far the least crowded of our 5 day stay. We did not have MM so were at DL; we were able to ride a lot before 11, including most of FL, Indy and others I can no longer remember. The parks did get more crowded as the day went on, but before noon it was really great. I was talking to a CM about this while waiting for rope drop and wondering aloud where everyone was ... she said the reason for it is because most guests at DL are locals, who often stay out late on Saturday and don't get up on Sunday mornings to go to the park.
Possibly the situation was different at DCA where MM was taking place so I would assume most guests there were not locals. But DL was great.