I've been 6 times in various months (May, June, Sept, Nov (2) and Dec and the best weather was in May and June. The temps were around 35 degrees and the only rain we had was at night. However as you're in the tropics you never can tell. One year in November we never saw a cloud for the whole holiday, yet alone any rain, but the next year at exactly the same tme we had 3 days of persistant, constant rain.
I would say avoid the months of Aug, Sept and Oct as these are the months with the highest risk of hurricanes - especially in the east of the island.