Hawaii is generally warm and inviting year round. Temperatures generally range from lows in the 70's to highs in the upper 80's. Because of the constant good weather, you can generally go to Hawaii anytime of the year.
Generally Hawaii always has a nice wind (unless your standing behind a HUGE building). This generally makes the temps in the mid to upper eighties wonderfully comfortable.
The coolest months are December through March (like the rest of the northern hemisphere).
The Hawaiian islands are full of microclimates - each island can have variances in rainfall from a few inches a year to 180 inches. Some locations, like Kaanapali Beach, rarely have a cloudy day, while others have many days. It is worth a little study to determine the best location to stay for your time of year. Some tips on good places to stay for generally sunshine infested days:
The US National Weather Service has a great deal of info, including forecasts, on the Hawaiian climate. Check it out at the link below.