I haven't been in twenty yrs., since the year after Epcot opened, so I've never experienced an EMH day personally. According to the Unofficial Guide, which I've read thoroughly for this trip, they should be avoided like the plague if you're not a Disney Resort guest. We're not - so I'm planning to go Sunday and Tuesday and possibly Thursday, as Wednesday and Friday are EMH days. Like I said, I really don't know if it makes a difference or not, but it makes sense... if I were a Disney resort guest and was offered extra hours over the general public, I would want to go on those days.
Hopefully, someone will post who has actually experienced MK on a EMH day in summer.