Anytime between Jan 1st and Dec 31st. Seriously tho, I've been there every month except Feb. and Oct. and have had a great time every time. It seems like its a little windier in Mar/Apr., and did have some rain in Nov/Dec but never for more than a day or two at most. The snorkeling/diving seemed best Mar-Sept IMHO, but that's only cuz I like it nice and warm! The people there are GREAT no matter when you go and that is the best part of Belize anyway! You'll love it!