The continental US is subject to dramatically shifting weather patterns, most notably produced by westerly winds sweeping across the continent from the Pacific. The best time to visit depends on where you're going and what you're planning to see. The Northeast, from Maine down to Washington DC, experiences low precipitation as a rule, but temperatures can range from bitterly cold in winter to uncomfortably hot and humid in the summer. Florida’s temperatures are not dramatically high in summer, but humidity is a problem; in the winter, the state is warm and sunny enough to attract many visitors.
The Great Plains are alternately exposed to seasonal icy Arctic winds and humid tropical airflows from the Gulf of Mexico. Winters around the Great Lakes and Chicago can be abjectly cold, and it can freeze or even snow in winter as far south as Texas, though spring and autumn get progressively longer and milder further south through the Plains. Tornadoes (or “twisters”) are a frequent local phenomenon, tending to cut a narrow swath of destruction in the wake of violent spring or summer thunderstorms.
In the South, summer is the wettest season, with high humidity, and the time when thunderstorms are most likely to strike. One or two hurricanes each year rage across Florida and/or the Gulf of Mexico states between August and October. The winter is mild for the most part and the two shoulder seasons usually see warm days and fresher nights.
Temperatures in the Rockies correlate closely with altitude, so nights can be cold even in high summer. Beyond the mountains in the south lie the extensive arid deserts of the Southwest. In cities such as Las Vegas and Phoenix, the mercury regularly soars above 100ºF, though the atmosphere is not usually humid enough to be as enervating as that might sound and air conditioning is ubiquitous.
West of the Cascades, the Pacific Northwest is the only region where winter is the wettest season, and outside summer the climate is wet, mild and seldom hot. Further south, California’s weather more or less lives up to the popular idyllic image, though the climate is markedly hotter and drier in the south than in the north, where there’s enough snow to make the mountains a major skiing destination from November to April. San Francisco and the northern coast is kept milder and colder than the inland region by its propensity to attract sea fog.
In addition to the main public holidays – on July 4, Independence Day, the entire country takes time out to picnic, drink, salute the flag, and watch or participate in fireworks displays, marches, beauty pageants, eating contests and more, to commemorate the signing of the Declaration of Independence in 1776 – there is a diverse multitude of engaging local events in the USA: arts-and-crafts shows, county fairs, ethnic celebrations, music festivals, rodeos, sandcastle-building competitions, chilli cook-offs and countless others.
Certain festivities, such as Mardi Gras in New Orleans, are well worth planning your holiday around but obviously other people will have the same idea, so visiting during these times requires an extra amount of advance effort, not to mention money. Halloween (Oct 31) is also immensely popular. No longer just the domain of masked kids running around the streets banging on doors and demanding “trick or treat”, in some bigger cities Halloween has evolved into a massive celebration. In LA’s West Hollywood, New York’s Greenwich Village, New Orleans’s French Quarter and San Francisco’s Castro district, for example, the night is marked by colourful parades, mass cross-dressing, huge block parties and wee-hours partying. Thanksgiving Day, on the fourth Thursday in November, is more sedate. Relatives return to the nest to share a meal (traditionally, roast turkey and stuffing, cranberry sauce, and all manner of delicious pies) and give thanks for family and friends. Ostensibly, the holiday recalls the first harvest of the Pilgrims in Massachusetts, though Thanksgiving was a national holiday before anyone thought to make that connection.
The state tourist boards can provide more complete calendars for each area.