The best time to visit Sint Maarten is mid-December through April, the dry season, when the trade winds keep temperatures comfortable (around 27°C/80°F), rain is infrequent, and the sea is calm. This is also peak season — prices are at their highest and the island is busiest, particularly around Christmas and New Year.
May through July is a shoulder period: slightly hotter and more humid, but prices drop significantly and the island is less crowded. August through November is hurricane season, with September and October being the most active months. Hurricane Irma devastated Sint Maarten in September 2017, causing catastrophic damage; the island has largely rebuilt, but the hurricane season risk is real and worth factoring in.
Avoid visiting when multiple cruise ships are in port in Philipsburg if you can — check the schedule in advance, as the difference between a ship day and a non-ship day is considerable.