The cool dry season (November to February) is the best time to visit Hpa-an. Temperatures are pleasant, the rice paddies are green and full, the roads are passable by bicycle, and the light is clear. The cave pagodas are accessible and the Zwegabin climb is not unpleasant.
March to May is hot — uncomfortably so — and the paddies turn brown. June to October is the monsoon season, which brings dramatic weather and flooding. Many roads into the surrounding countryside become difficult or impassable in heavy rain, and the caves can be partly flooded. However, the landscape is intensely green and the Thanlwin River runs full.
If visiting during the monsoon, expect to be more dependent on local transport and less able to cycle freely.