I don't have enough experience to back this up, but I am a BIG believer in following "recommended park days" at any time of the year. I like reading the crowd calendars and detailed explanations for the recommendations on EasyWDW and keeping those in mind as much as possible when planning which park/which day. Again, not enough real world experience to back this up, but I am convinced that MK is a year-round MADHOUSE for crowds, and the first thing I did when planning my park days for this year was look at EasyWDW to see which days were "green" (best) for MK and working everything else around that.
I still have memories of my first trip in early Dec '99 (back in the "good old days"), when the only real crowd we saw was Saturday night at MK, and it was nuts! From that point on I have refused to set foot in MK on a Saturday. (That night, we bailed on MK and hopped to Epcot, which was a magical oasis of tranquility - seriously.)