The ideal time to visit the islands is from December to May because during these months the weather is dry and warm. The warm temperatures last all year, but this is the best time to go because of other weather issues. August and September should be avoided because this is the time for hurricanes to arrive, but this could also be one of the cheapest times to visit.
Also in June and July and again in October and November the islands experience a high amount of rain and a huge amount of humidity. These conditions can make sightseeing and exploring difficult, but still doable. However, this is another time when you will be able to find good deals on airfare and rooms since the end of November is the time when all of the tourists go home.