We're going on a Monday when we visit in Sept but i think it will be busier than the weekend for similar reasons to others on here (new arrivals wanting to start at MK). I think this is backed up by the opening hours, there are no EMH's on that day but the park is open until 9pm with Spectromagic and Wishes unlike the rest of the week when the park closes at 7/8pm with no Spectro or Wishes...but we really want to see these so i don't mind it being a little busier...I know it shouldn't be as busy as July when we went last year. Plus we're spending half another day in MK for our Halloween party (we're arriving at around 2.30 so we can enjoy rides etc during the parade and before the party starts) so we'll get it all in.
Might be worth looking at the opening times tho...I suspect if Mondays are generally busier you should see some changes in opening times regularly!?...There must be a reason why it's open later with Spectro and Wishes on a Monday when we are there.