Anytime of year is a good time to visit Hawaii. The average temperature here is between 25I- 32I C. Summer, between April and November is warmer and drier while winter, between December and March, is a bit cooler. Trade winds keep things comfortable year-round.
Depending on where you're located in Hawaii, the weather can be very different over even short distances. On the same day, on Oahu you might find sun over the beaches in Waikiki and rain only a few miles away in Manoa Valley.
Although there are no true "seasons" in the islands in the same sense as the rest of the U.S., the climate does go through annual cycles based on rainfall. The "wet" season in Hawaii (cooler temperatures and more rainfall) runs roughly from October to March, and the "dry" season (warmer temperatures and less rainfall) from April to September. There is therefore a higher probability of rain if you visit during the peak of tourist season in late December or January.
Overall, Hawaii is warm and balmy - when you step out of the plane you'll immediately notice that the air is soft and humid - and during the summer months the trade-winds provide a pleasant breeze.