I'm up in the Pocono mountains of Pennsylvania. It's generally cold from December through March here. Snow that fell in January is still on the ground here in late February. This is our time to escape to WDW.
This year we went in December. The first week was lovely, 75 degree days. The second week wasn't bad, 65 degree days. In past December trips, we've experienced days where it was was a high of 50, and a day where the high was 85.
In many years we go in late January or early February. Generally, high temps are in the low 70's. Coming from a colder climate, we're in the heated pool with the folks from Canada, Minnesota, Wisconsin, and Maine, and comparing the weather at home with the lovely Florida weather we're experiencing.
We've gone in March for Flower and Garden, and found the temperatures to be in the mid to high 70's. Also a lovely time to go.
We made the mistake of going in September once. It was over 90 with 90% humidity. We couldn't take it. Our day began at rope drop and we left the park at lunch time to avoid the heat. We came back to the parks after the sun was down and the pavements cooled off.
In the winter, you can pick out the Floridians from the folks from the north very easily. The Floridians are wearing parkas and the northerners are wearing T shirts and shorts, in 60 degree weather.