Hawaii has two seasons: summer (May to October) and winter (November to April). The difference is modest in terms of temperature — both are warm — but significant for rain, surf, and crowds. Summer is drier, with the North Shore waves flat and calm for swimming. Winter brings the big surf to the North Shore (Pipeline and Sunset Beach are at their best November through February), more rain in the valleys, and whale watching season — humpback whales pass through Hawaiian waters from December through April.
December through March is the peak season for North American and Australian visitors, which means high hotel prices and crowded beaches. If you can visit in April-May or September-October, prices drop and crowds thin without losing the good weather. Avoid Chinese New Year and Golden Week (late April) if you dislike queues at popular attractions.