Any time EXCEPT the week around Thanksgiving (including both weekends) and the last half of December should be great. We went 2nd weekend of December last year and it wasn't bad at all. Be aware that some park hours are shortened due to the light crowds, though, too. It was good for us because DS2 could see the fireworks ~6PM, but if you have older kids you might want to plan around Extra Magic Hours. I think the hours are already posted on disneyworld.com.