Since the United States spans thousands of miles, there is always a good time to visit.  If you don't like the weather in one part of the country, you can simply go elsewhere. 

If it's hot weather you're looking for, head down south.  States around the Gulf of Mexico experience warm weather year-round, like most of the southern states.  Hawaii is, of course, another popular choice for places to get a tan.

Cooler/colder weather can be found in the nothern states, especially during the winter months (December through March) when you will likely experience crisp cool air and the occasional snow storm depending on where you are. 

The Great Lakes region gets plenty of snow during the winter (due to the lake effect), as does Vermont, New Hampshire, Maine, and upstate New York and the mountains of the West.  Western Massachusetts will usually get more snow than Boston and coastal Massachusetts.  However, it can get hot in these areas during the summer and most Bostonians and New Yorkers can attest to the fact that humid weather really gives the heat an extra kick. Even Alaska gets a break from the cold weather and snow in the summer, when the days get really, really long.