For future reference, I think Sunday is now the worst night. Limited hours but seemingly as crowded as any other night--and big lines from opening.
In contrast, Fridays tend to be surprisingly good, at least at the start and end of the night. Might be the higher ticket prices, who knows, but crowds take a while to build and are not necessarily the worst you'll see. If you can be there at opening, maybe get dinner at CityWalk or at least walk the scare zones and see the shows from 9 - 12, and then return to houses from 12 - 2, you're all but guaranteed of seeing everything.
Early Thursdays are good. But once Orange/Osceola/Seminole County schools start getting out on Fridays mid-month, very bad.
Wednesdays tend to be bad for the same reason (they generally are only scheduled in "hell weeks"). And again the limited hours, closng at midnight.
Saturdays remain and will always be bad, except maybe the last Saturday before Halloween.