Most visitors don't come to Hawaii when the weather's best in the islands; rather, they come when it's at its worst everywhere else. Thus, the high season -- when prices are up and resorts are booked to capacity -- generally runs from mid-December through March or mid-April. The last 2 weeks of December in particular are the prime time for travel to Oahu; if you're planning a holiday trip, make your reservations as early as possible, count on holiday crowds, and expect to pay top dollar for accommodations, car rentals, and airfare.
The off seasons, when the best bargain rates are available, are spring (mid-Apr to mid-June) and fall (Sept to mid-Dec) -- a paradox, since these are the best seasons in terms of reliably great weather. If you're looking to save money, or if you just want to avoid the crowds, this is the time to visit. Hotel rates tend to be significantly lower during these off seasons. Airfares also tend to be lower -- again, sometimes substantially -- and good packages and special deals are often available.
Note: If you plan to come between the last week in April and mid-May, be sure to book your accommodations, interisland air reservations, and car rental in advance. In Japan, the last week of April is called Golden Week, because three Japanese holidays take place one after the other; the islands are especially busy with Japanese tourists during this time.
Due to the large number of families traveling in summer (June-Aug), you won't get the fantastic bargains of spring and fall. However, you'll still do much better on packages, airfare, and accommodations than you will in the winter months.
The Weather--Because Hawaii lies at the edge of the tropical zone, it technically has only two seasons, both of them warm. There's a dry season that corresponds to summer, and the rainy season generally runs during the winter from November to March. It rains every day somewhere in the islands at all times of the year, but the rainy season can cause "gray" weather and spoil your tanning opportunities. Fortunately, it seldom rains for more than 3 consecutive days.
The year-round temperature usually varies no more than 10°F (-12°C), but this does depend on where you are. Oahu is like a ship in that it has a leeward and a windward side. The leeward side (the west and south, from Waianae to Honolulu and Waikiki) is usually hot and dry, while the windward side (east and north, from Haleiwa to Waimanalo) is generally cooler and moist. When you want arid, sun-baked weather, go leeward. When you want lush, often wet, junglelike weather, go windward.
Hawaii also has many microclimates, thanks to its interior valleys, coastal plains, and mountain peaks. It can be pouring rain at the University of Hawaii in Manoa, and sunny and dry in Waikiki, a 15-minute drive away.
The best months to be in Hawaii are April, May, September, and October, when the nearly perfect weather is even better -- not too windy, not too humid, but just right. And this is the off season: The kids are in school, and the tourists have thinned out. The state's "carrying capacity," as they say here, isn't maxed out. Hotels, restaurants, and attractions aren't as crowded, and everyone is more relaxed.
On rare occasions, the weather can be disastrous. The official hurricane season is June to November. (Remember Hurricane Iniki in Sept 1992? It was the most powerful Pacific storm in history, and it crushed Kauai with 225-mph winds.) Tsunamis, huge tidal waves caused by far-off earthquakes, have swept the shores of the islands. But those are extreme exceptions. Usually one sunny day simply follows another here, each quite like the other.
Holidays--When Hawaii observes holidays, especially those over a long weekend, travel between the islands increases, interisland airline seats are fully booked, rental cars are at a premium, and hotels and restaurants are busier than usual.
Federal, state, and county government offices are closed on all federal holidays: January 1 (New Year's Day); third Monday in January (Martin Luther King Jr. Day); third Monday in February (Presidents' Day, Washington's Birthday); last Monday in May (Memorial Day); July 4 (Independence Day); first Monday in September (Labor Day); second Monday in October (Columbus Day); November 11 (Veterans' Day); fourth Thursday in November (Thanksgiving Day); and December 25 (Christmas).
State and county offices also are closed on local holidays, including Prince Kuhio Day (Mar 26), honoring the birthday of Hawaii's first delegate to the U.S. Congress; King Kamehameha Day (June 11), a statewide holiday commemorating Kamehameha the Great, who united the islands and ruled from 1795 to 1819; and Admission Day (3rd Fri in Aug), which honors Hawaii's admission as the 50th state in the United States on August 21, 1959.
Other special days celebrated by many people in Hawaii but that do not involve the closing of federal, state, or county offices are Chinese New Year (in Jan or Feb), Girls' Day (Mar 3), Buddha's Birthday (Apr 8), Father Damien's Day (Apr 15), Boys' Day (May 5), Samoan Flag Day (in Aug), Aloha Festivals (in Sept or Oct), and Pearl Harbor Day (Dec 7).
Hope this helps!