The Thai climate has three seasons. The hot dry season runs from February to April, the wet season from May to October, and the cool dry season from November to January.
November to February is the best time to visit in pure weather terms — clear days, low humidity, daytime temperatures in the high twenties. The pilgrimage at Khao Khitchakut runs in this window.
May to July is wet and humid, with strong afternoon storms, but it is also durian and rambutan season — the fruit markets at their best and the orchards open for visits. If you came for the fruit, this is the only time. The waterfalls run at full strength.
Weekends are when the gem market is open, so plan a visit for Friday to Sunday if that interests you.