The months of July to September should should be avoided since in that time hurricanes are a common thing there. You won’t be able to enjoy yourself or you may even not be able to reach the place. You can face a lot of problems if you go travelling at that time within these islands.
It is also not that good an option since heavy rains may dampen any plans you may have or sitting on the beaches or exploring the wildlife. The other times of the year are perfectly okay while travelling to the islands. You will get good weather and face no problems. Unless you come across a sudden hurricane but those cannot be predicted. That’s a very rare case so you don’t really have to worry about that.