There are many great times to visit Hawaii. In fact, any time of year is great. But, few months are better than others for different reasons. April, May, September, and October and the best months where the weather is concerned. It can get hot in Hawaii and these months tend to be the mildest. If you are looking for the best rates to visit Hawaii, January and February are usually the cheapest. You can get great air fares around that time too.
It is generally consiered more of an off season rate. The weather is usually always good in Hawaii so that is really not too much of a factor. The winters are very mild and the summers can lean to the hot side but if you are enjoying water sports, you might prefer for it to be hot. The best time of the year to go heavily dependson what do you want to do while you are there. There are seasons for whale watching and seasons for most everything else. There really is no bad time to visit Hawaii, really.
When there are holidays and large events, prices in Hawaii rise dramatically. The best time to visit may be in early Spring when the trade winds have lessened and also the rain. The weather is perfect and will not disappoint. If you go before the last week in April you will miss the crowds which occur then for a fortnight because it is the time when typically Japanese tourists visit.