Cha-am's climate is shaped by two monsoon systems and is arguably the most reliable on the Gulf coast. The town sits in a rain shadow of sorts, catching less rainfall than Koh Samui or the east coast further south.
The best months are November through February: dry, with daytime temperatures around 28-30°C and pleasant sea breezes. March through May are hotter and increasingly humid but still dry. The southwest monsoon brings rain from June through October, though Cha-am receives far less than the Andaman coast during this period. Even during the wet season, storms tend to pass quickly and the beach is often usable between downpours.
Avoid the Songkran holiday (mid-April) if you want peace — the town fills with Thai families for the water festival and accommodation prices double. The long Thai holiday weekends in May (Labor Day, Chakri Day) also bring crowds. Weekdays at any time of year are quieter than weekends, as most Thai visitors come on two-day escapes from Bangkok.