Long days of sunshine and fairly mild year-round temperatures make Hawaii an all-season destination. Most resort areas are at sea level, with average afternoon temperatures of 75°F–80°F during the coldest months of December and January; during the hottest months of August and September the temperature often reaches 90°F. Only at high elevations does the temperature drop into the colder realms, and only at mountain summits does it reach freezing.
Kauai is beautiful in every season, but if you must have good beach weather, you should plan to visit between June and October. The rainy season runs from November through February, with the windward or east and north areas of the island receiving most of the rainfall. Nights can be chilly from November through March. Rain is possible throughout the year, of course, but it rarely rains everywhere on the island at once. If it's raining where you are, the best thing to do is head to another side of the island, usually south or west.
If you're a beach lover, keep in mind that big surf can make many North Shore beaches unswimmable during winter months, while the South Shore gets its large swells in summer. If you want to see the humpback whales, February is the best month, though they arrive as early as October and a few may still be around in early April. In winter, Napali Coast boat tours can be rerouted due to high seas, the Kalalau Trail can become very wet and muddy or, at times, impassable, and sea kayaking is not an option. If you have your heart set on visiting Kauai's famed coast, you may want to visit in the drier, warmer months (May–September).
If you happen to be in the Islands on March 26 or June 11, you'll notice light traffic and busy beaches—these are state holidays not celebrated anywhere else. March 26 recognizes the birthday of Prince Jonah Kuhio Kalanianaole, a member of the royal line who served as a delegate to Congress and spearheaded the effort to set aside homelands for Hawaiian people. June 11 honors the first island-wide monarch, Kamehameha I; locals drape his statues with lei and stage elaborate parades. May 1 isn't an official holiday, but it's the day when schools and civic groups celebrate the quintessential Island gift, the flower lei. Statehood Day is celebrated on the third Friday in August (admission to the Union was August 21, 1959). Check the daily Garden Island paper for other events.