The most comfortable time to visit the region is during the dry season between May and October. Not surprisingly, this is when most of the major festivals are held. It is also considered to be the region’s high season, though there isn’t necessarily much difference in costs.
The wet season (ostensibly its low season) lasts from November to April. March tends to be the wettest month, especially in Vava’u. Remember, though, that most precipitation occurs at night and the main discomfort will be caused by a rise in the lethargy-inducing heat and humidity. Tonga lies squarely within the South Pacific’s notorious cyclone belt. The season for tropical storms and cyclones is between November and March. Cyclones seem to occur, on average, every 10 to 15 years.
If you plan to come during the December-January holiday period, when huge numbers of Tongans return for the holidays (mostly from New Zealand, Australia and the USA), it’s wise to book flights well in advance.