The dry season runs from November to April — this is when Koh Lanta is at its best. The sea is calm, the sky is clear, and the visibility for snorkelling and diving is excellent. Peak season is December to February, when European visitors arrive in numbers and prices rise accordingly. February and March are the sweet spot: still dry and sunny, but cheaper and less crowded.
The monsoon season runs from May to October. Rain comes in heavy bursts rather than all-day drizzle, and some days are perfectly sunny. However, the sea becomes rough, ferries become infrequent or stop entirely, and many resorts close for the season — particularly in the July–September trough of the monsoon.
If you come in May or October (the shoulder months), you'll find a quieter, cheaper island with good weather on most days, though you need to accept that some facilities will be closed and the ferry connections will require planning.