Hawaii has mild weather year-round, which is its greatest selling point. Average temperatures hover around 77 degrees Fahrenheit, ranging from about 72 in January to 82 in July. Trade winds keep the heat comfortable most of the time.
Dry season (April to October) brings the best weather and the biggest crowds. This is peak season, especially during summer and around holidays. Hotel and flight prices are highest.
Wet season (November to March) means more rain, particularly on the windward (northeast) sides of the islands. The leeward sides stay much drier. Winter brings bigger surf to the north shores -- the North Shore of Oahu becomes a world-class surfing destination. Humpback whales migrate to Hawaiian waters from December through May, with peak sightings in January and February. The best spot for whale watching is off the coast of Maui.
Shoulder months (April-May and September-October) offer a good balance: decent weather, fewer crowds, and lower prices.
Keep in mind that each island has many microclimates. The windward sides are lush and rainy; the leeward sides are dry and sunny. Altitude matters too -- Haleakala on Maui and Mauna Kea on the Big Island can be genuinely cold, with temperatures dropping below freezing at the summit.