We often go in December. One trip it rained 7 days out of 8 and that was not just a little rain, some days it poured down that year. We just carried on. I'm not going to spend my vacation, especially at Disney, sitting in a hotel room. But on the other hand, we've also had it be 85, or below freezing. Sometimes on the same trip. I always recommend to anyone going in December to pack a little of everything and then layer if you have to. I agree with raincoats over ponchos for that extra layer. Also poncho's blow around if it gets windy. Always find Epcot to be particularly windy, especially around Spaceship Earth.