Southern Thailand Islands in September

The sweet spot runs November through April when both coasts enjoy dry weather and calm seas. December and January are peak season – expect crowds and higher prices but also the most reliable weather for island hopping. March and April get hot, really hot, but the diving is exceptional and you'll have more beaches to yourself. Here's where it gets tricky: the two coasts have opposite monsoon seasons. The Andaman side (Phuket, Krabi, Phi Phi) gets hammered from May to October, while the Gulf side (Samui, Phangan, Tao) stays relatively dry until November. But monsoon season isn't necessarily a write-off – accommodation prices drop by half, and you'll experience a different side of island life. Just know that some ferries stop running, and outdoor activities get cancelled. If you're planning around Full Moon Party season, book Koh Phangan accommodation months in advance and prepare for inflated prices across the entire region.