Best time to go to United States of America
The sheer size of the United States makes the country a year round destination. One part or another always has fantastic weather, though conversely, another is guaranteed to be awful. The trick is to know your destination and season.
Most visitors come to America in the summer months, which here is defined as starting on Memorial day (last Monday in May) and finishes up on Labor Day (first Monday of September). This is also the time most Americans vacation, as schools are out, rather than the weather being the best. In fact, quite often the destination you're interested in will have a much nicer climate in Spring or early Autumn.
Spring
Spring, during March to May, is an ideal time to visit the major cities like San Francisco, Los Angeles, Miami, New York, or Chicago. Later on, the heat is oppressive and humidity can hit 100%! Florida is also extremely pleasant during this time, especially in the Gulf of Mexico part, where you can swim in the warm waters. The same goes for the Texas coast line and South Padre island.
Summer
Summer months are burning hot in the desert states of New Mexico and Arizona, and the Midwest is so humid that walking outside feels like a sauna. However, it's the perfect time to visit the Pacific Northwest as there is little rain. The mountain states, Colorado, Wyoming, Montana, and the high elevations parts of Arizona and Utah are great during the summer time. Temperatures are moderate, there is plenty of sunshine, and the rain stays away.
Fall
September to November is the normal fall season in the United States, and again the major cities are pleasant destinations. For beautiful foliage, visit the New England states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut. This is also a good time to visit much of the Midwest, as the harvests are in full swing, there are food markets everywhere, and the weather stayps pleasant most of the time.
Winter
Ski season kicks off in November through March for much of the mountain states of Colorado, Utah, Wyoming, and Montana and it can be quite expensive. Aspen in Colorado has some of the world's best skiing, but also at some highly elevated prices to go with the high altitude slopes. Somewhat cheaper choices can be Steamboat and Jackson Hole. Winter is also expensive in Florida due to migrating 'snowbirds' and is high season for Hawaii.
Festivals
There are literally far too many to mention. Make sure to plan around festivals and big conventions though, as hotel room rates shoot up and restaurants get packed to capacity in many locations. Las Vegas especially can be affected, as is Austin at anytime that SXSW, or South by Southwest, takes place. For more about festivals, look at the guides for each state and city in America.
