I was there last year on the first weekend of December. It was very busy when I was there (well it was a weekend), and I remember trying to do some last minute shopping on main street just before the parade on the Sunday and could barely move, there were so many people on main street and in the shops!
But - it was very quiet on Saturday night (after dinner time - the park closed at 10pm), and I walked on to a number of attractions after dinner including PoTC and Indiana Jones - both of which had nearly hour long queues earlier in the day. EMH on Sunday was also quite quiet, the crowds only really started getting in after 10 or 11am.
So possibly the Sunday when you're there might be busy? The weekdays are apparently less so though...