Climate differences are dramatic across the United States. When it's bone cold in New England, the upper central states, and Alaska, it's sunny and warm in Florida, California, and Hawaii. When it's raining cats and dogs in the Pacific Northwest, it's dry as, well, a desert, in the Southwest desert. It can be a pleasant 75°F (24°C) on the beaches of Southern California in summer, yet 120°F (49°C) a few miles inland.
This works out well for travelers, because there isn't a nationwide high or low season: It's always shoulder season somewhere. In summer, room rates are highest on the Northeast and mid-Atlantic beaches but lowest on the sands of hot-and-humid Florida (though not in Central Florida, where rates can go sky-high in the Land of Theme Parks) and in the sticky climes of the Gulf South. Winter snows virtually close the great Rocky Mountain national parks and the major tourism centers of the northern Great Plains, but bring crowds to the nearby ski slopes. Alaska is usually well below freezing until summer, when the midnight sun smiles down on warm days and higher hotel rates greet the tourist crowds. Hawaii is warm year-round, but winter brings massive amounts of rain -- and higher prices.
The Northeast and mid-Atlantic states have their summer beach season from June to Labor Day and their great fall foliage in September and October. Climate can vary wildly in these regions: One day can be warm and lovely, the next muggy and miserable. Winter storms are not an infrequent occurrence in these regions -- New England is famous for its nor'easters -- though some mid-Atlantic winters in the not-too-distant past have been very mild (and others have been frigid).
Summer can be brutally hot and humid in the Southeast (and is also prime hurricane season), but spring and fall last longer there, and winter is mild -- with snow the exception rather than the rule. The Gulf South summers are often exceptionally hot and humid, though winters (except in the mountain areas) are generally mild, if rainy. Southern Florida's best season is from January to April, though cold snaps can turn it nippy for a few days.
The central states see harsh winters and scorching summers. Southwest weather varies from east Texas's hot, humid summers and mild winters to Arizona's dry, 110°F (43°C) summers and pleasant, dry winters. Nevada is similar, though it tends to get a bit chillier in winter. The mountains of Colorado, Utah, and the Northwest have dry, moderately hot summers and cold, snowy winters. The California coast is fine all year except early spring, when it rains; the Northwest coast is wet most of the time except July.
The long and the short of it: Late spring and early fall are the best times to visit most of the country.