The Grenadines sit outside the main hurricane belt, so Mustique escapes the worst of Atlantic storm season (June to November), though heavy rain and rough seas can still occur. The driest and calmest months are December through May — this is also high season, when villa rentals peak and Basil's Bar is at its liveliest.
January and February are the height of it: cooler trade winds, little rain, and full occupancy. March and April remain good. May starts to get warmer and quieter. June through November is officially low season, with cheaper villa rates and fewer visitors — the sea can be rougher and some facilities reduce hours, but the island itself is no less beautiful.
Turtle nesting season runs from May to October at Pasture Bay. If you want to watch hawksbill turtles come ashore at night, the warmer months are when to come.