Vacation in the USA in December can be quite deservedly called comfortable. Although the weather varies greatly from state to state, in general, winter in the USA is mild. Except, of course, Alaska, where the temperature in December reaches -10°C. In the rest of the country you will not meet during a vacation in the U.S. in winter temperatures below +4°C. And in Florida the air temperature does not fall below +22°C, and it is quite possible to swim here.
In the mountains in winter it is much colder. Ski tours to the USA in December are especially popular. The slopes of the Canyons, Aspen and other resorts are invariably full of tourists.