Being a huge country, the United States is home to a wide variety of climates. However, in general it has a continental climate, with cold winters (often frigid) and hot summers (sometimes very hot), more or less continental depending on the latitude and the distance from the sea. There are, however, some exceptions: on the west coast overlooking the Pacific Ocean, the climate is cool and damp in the northern part and Mediterranean in the southern part; on the coast of the Gulf of Mexico the climate is mild in winter and hot and muggy in summer, while in Florida it is almost tropical; the mountainous areas are cold in winter and cool or cold even in summer; and finally the deserts are mild in winter and scorchingly hot in summer.
Since there are no obstacles to the descent of cold air from Canada, almost all of the country can experience sudden cold waves in winter, but they have different durations depending on area: they last a few days in the south, and lower the temperature a few degrees below freezing (0 °C or 32 °F) in winter, while they are intense in inland areas, in the highlands and in the north-east. The summer heat waves can be intense as well, especially in inland areas. In general, the western half of the country is more arid than the eastern one, with the exception of the north and central coast of the Pacific, which is wet.

United States: division between west and center-east

See the climate of:
United States West
United States Center East

Best Time

It's hard to find a single period in which the weather is very good throughout the country. In general, however, since the climate of most of the United States is cold in winter and hot in summer, you can choose spring and autumn. It must be said, however, that during spring tornadoes are possible in the inland areas of the central and eastern part, and in autumn hurricanes can affect the southernmost area, still of the center-east.

Being that it's mild even in summer, you can visit the west coast, including Seattle and San Francisco, from May to September. On the coast of Southern California and Los Angeles, you can go throughout the year, but they are preferable in spring, from March to May; in this way, you can move to the interior without fear of the heat (but in the desert the month of May can be already hot), or to the parks of the Sierra Nevada without fear of the winter cold.
You can visit the Death Valley from November to March, however, the best months are February and March (in this way you can reduce the risk of the cold at night). This period is generally recommended for the hot desert areas.
You can visit the Grand Canyon, in April and October, in order to avoid both the summer heat in the lower part and the cold of the upper part. In the high mountain areas of the Rockies, including the Yellowstone National Park, you can go in summer, from June to August.
The best periods in New York are the month of May, and from mid-September to mid-October; the latter is generally a mild and relatively calm period. In winter, there's the risk of bitter cold and snow storms. In summer, sometimes there can be an uncomfortable heat, while thunderstorms can break out in the afternoon. In early spring, the weather can be unstable, ad sometimes cold.
This period of early autumn is good throughout the north and north-east, but also in the highlands of the western area, bearing in mind that at night it can be already cold. May is also a good month in these areas.
You can visit Miami, the Everglades and Southern Florida from November to April, but for a beach holiday you can choose the period mid-March to late April, in order to avoid the risk of the rare but not impossible cold periods.
March-April and October-November are good periods to visit the southern regions and those bordering the Gulf of Mexico. In the former, however, tornadoes are possible away from the coast, while in the latter, especially in October, hurricanes are still possible. In winter, the weather is often pleasant, but we cannot exclude short cold waves.

What to pack

In winter: in the northern and mountainous areas, pack cold weather clothing, synthetic thermal long underwear, a down filled parka with insulated hood, gloves, warm boots, winter hat; T-shirts and light shirts for hot indoor environments.
In New York, pack a down jacket, gloves, a scarf, a hat.
In the Seattle area and the north-west coast, pack warm clothes, winter coat, a raincoat or umbrella.
In Los Angeles and Miami, clothes for spring and autumn, an umbrella.
In the southern states and northern Florida, clothes for spring and autumn, but also warm clothes and a warm jacket for short cold spells.

In summer: in the northernmost areas, pack light clothing, T-shirts for hot days, a jacket and sweatshirt for the evening and cool days, a raincoat or umbrella.
On the west coast, down to San Francisco, clothes for spring and autumn: a T-shirt, a shirt, a jacket, a sweatshirt or sweater.
In New York, Chicago, Boston and Washington: light clothing, a T-shirt, a jacket and a sweater for the evening and cool days, an umbrella, a sweatshirt and pashmina for airconditioned places.
In desert areas and all of the southern belt: lightweight and loose-fitting clothing, a sun hat, a sweatshirt and pashmina for airconditioned places, comfortable and breathable shoes; in Florida and the southern states, an umbrella or lightweight rain gear; in the deserts, a light scarf for the wind full of dust.
In mountainous national parks: hiking shoes, a jacket and sweatshirt or sweater for the evening, sunscreen, sunglasses, hiking boots, an umbrella or raincoat.

Other resources available for the climate of the United States and of the overseas possessions:

American Samoa (Oceania)
Hawaii (Oceania)
Puerto Rico (Caribbean)
United States Virgin Islands (Caribbean)