I went the first week in January several years ago. We were lucky enough to have weather in the 80's, but that it not always the case. I've heard that it can be anywhere from the 30's to the 80's, so it's hard to plan. The only thing that I would caution you to avoid is the week of the marathon. We didn't do our homework ahead of time and were there for the WDW marathon, along with all the runners and their families. It's usually the first weekend in January. The crowds were moderate, but if you're looking for a time with light crowds, I think you could go one of the other three weeks in January or sometime in February (obviously not President's week) and have signifigantly smaller crowds. Another problem that we had on the actual day of the marathon was that the bus system was completely rerouted so as not to cross the marathon path. It took us an hour and a half to get from the CBR to the MK as a result.