Anytime of year is a good time to visit Hawaii. Summer, between April and November is warmer and drier (average temperature is 75˚-88˚ F) while winter, between December and March, is more temperate (68˚-80˚ F). Trade winds keep things comfortable year-round.
I would suggest bringing a sweater or light jacket for the evenings. The winds can be a bit breezy depending on where you are.