I went in early October last year and I loved it, the decorations for Fall and Halloween were great. I went to
MNSSHP and I really loved the parade. I have been to
MVMCP several times so a different themed parade was quite a treat.
The wait for the lines was about 10 minutes most times, with a lot of the rides being walk on. Of course this was not true for Peter Pan and Soarin, but generally the wait time was minimal.
I've been pretty lucky with the weather in December too, this Dec the weather was in the mid to low 80s, a fluke I know but I wore tee shirts and shorts the whole time. In past years I've changed to jeans and taken a sweatshirt for the evenings, but usually you can wear shorts and tees during the day.
The first few weeks of December are getting more and more crowded, each year I have seen an increase in the crowds. So I think the secret about it being a slow time of year has gotten out But I do enjoy the Christmas decorations.
So far though I think October is still a less busy time to go, so decide which decorations you prefer and have a great time.