The dry season runs roughly May to October, when temperatures stay around 22–27°C and humidity is bearable; this is when most visitors come. November to April is the cyclone season — hotter, wetter, with occasional heavy storms — though the rain rarely lasts more than a day.
June to October is humpback whale season. Whales calve in the warm shallows around Tongatapu and Vava'u; Tonga is one of the few countries in the world where licensed operators are permitted to swim with them. September brings the Heilala Festival, a week of competitive Tongan dancing, beauty pageants, and the king's birthday celebrations.
Note that nothing opens on Sundays. The constitution declares Sunday a day of rest; shops, restaurants, and public transport are mostly closed. Hotels keep running and a few cafes are open for guests, but plan ahead.