We've been this week several times. Crowds get lighter as each day goes by.
You will run into Mousefest, which can be fun, toward the end of the week. (It doesn't really seem to effect crowds, though attendance gets bigger every year) Also, they film the Christmas parade at MK in there somewhere, so you'll want to avoid that park that day.
If you have the option, we have been the second week in December too, and it is even better than the first week. But both are great times to go.