We were there the same week in 2006 - Dec 11th thru 16th (a Monday to Saturday). The crowds were fine. Not as low as Sept or Feb, but very managable. Really didn't wait for much except to meet princesses and some of the shows. We rode everything multiple times - in fact our Epcot day we called "Double Day" because we rode everything two times in a row. We had very good restaurant experiences by eating at off-peak times. Friday, I noticed the crowds starting to pick up and Saturday even more. The weather was nice - 60s/70s & tshirts during the day and fleece jackets at night. It did rain on us once. I'm very cold natured so I would never swim at those temps but I'm sure others might. (I'm from Texas so I like it to be HOT when I swim!)
It's probably the lowest crowds of the whole month. The first week you have parade taping and pop warner crowds. Then after the weekend of 15th/16th crowds will steadily increase until Christmas.