Hawaii is sunny and warm year round, so whenever you decide to visit the islands, it is considered to be a good time. However, even warm year round, the weather here varies a bit during the seasons.
Hawaii is generally very humid and humidity could be feel most during the month of October, right before the winter starts, so if you don’t like humidity, that’s not the time to plan a Hawaii vacation.
The summer months May to August are usually busy on the islands. That’s the time when many tourists plan their Hawaii vacation. The most busy of all gets the island of Oahu and especially Waikiki. One evening in August while I was in Waikiki, I wanted to buy a bottle of water from an ABC store and I was shocked to see the store was entirely packed with people and I even was not able to get it. So if you would like to enjoy a Hawaii vacation in Waikiki between May and August be prepared to see many tourists. Often times you may also need to wait from 20 to 60 minutes to get sited in restaurant. The other islands of Maui, Kauai and the Big Island are overall not so busy, so you may want to consider them for your summer vacation.
Christmas time also gets pretty busy in Hawaii. Also flights to the islands are more expensive around the holidays.
June to November is the hurricane season in Hawaii. If you plan vacation during that time, don’t let this scare you off. Hurricanes hardly ever occur in Hawaii. And even if one hits Hawaii, the state is prepared to accommodate visitors and citizens.
Personally I would recommend February to April as a best time to visit. That’s the end of the winter in Hawaii and you can enjoy nice sunny weather and less humidity.