The dry season — late November to April — is the only practical time. The Cardamom Mountains catch some of the heaviest rainfall in mainland Southeast Asia, and the wet season here is genuinely wet: jungle trails become rivers, roads wash out, and many of the river and beach trips simply do not run.
December to February has the most comfortable temperatures (around 25-28 C in the day) and the calmest seas for the boat trip to Koh Kong Island. March and April are hot — well into the high thirties — but still dry, and the waterfalls are at their lowest, which is ideal for swimming.