Koh Rong in January

November through April is peak season for good reason. The northeast monsoon keeps things dry, temperatures hover around 28°C, and the sea stays calm for boat transfers. December and January are busiest — book accommodation well ahead. May through October brings the southwest monsoon. Expect daily afternoon downpours, rough seas that cancel boat services, and many guesthouses closing entirely. But here's the upside: the few places that stay open offer serious discounts, and you'll have beaches almost entirely to yourself between rain showers. The bioluminescent plankton peaks during the dry season, especially on dark nights between November and March. Full moon periods wash out the effect, so time your visit for the new moon if this is your main reason for coming. Avoid Cambodian holidays like Khmer New Year (April) and Pchum Ben (October) when domestic tourists pack the island. Chinese New Year (January/February) also brings crowds, though the atmosphere stays more international than local holiday periods.