Tongatapu is far enough south of the equator to have cool, dry, and quite pleasant weather during the austral winter months (July-Sept), when temperatures range between 60°F and 70°F (16°C-21°C). However, the ends of occasional cold fronts from the Antarctic and periods of stiff southeast trade winds can make it seem even cooler during this period. During the summer (Dec-Mar), the high temperatures can reach above 90°F (32°C), with evenings in the comfortable 70s (20s Celcius). A sweater, jacket, or wrap will come in handy for evening wear at any time of the year.
The islands get about 180 centimeters (71 in.) of rainfall a year, the majority of it falling during the summer months. Vava'u to the north tends to be somewhat warmer and slightly wetter than Tongatapu.
Tonga is in the southwestern Pacific cyclone belt, and hurricanes are possible from November to April. There will be ample warning if one bears down on the islands while you're there.
Holidays & Events
The largest annual festival is Heilala, the week of July 4, when Nuku'alofa goes all out for a week of dance and beauty competitions, parades, sporting matches, concerts, marching contests, regattas, parties, and the lovely Night of Torches on the waterfront. Tongans living overseas like to come home for Heilala, so hotel reservations should be made well in advance. Vava'u stages its own version of Heilala early in May.
The Tonga Visitors Bureau keeps track of festival dates.
Public holidays are New Year's Day, Good Friday and Easter Monday, Anzac (Memorial) Day (Apr 25), Independence Day (June 4), the King's Birthday (Aug 1), Constitution Day (Nov 4), King Tupou I Day (Dec 4), Christmas Day, and Boxing Day (Dec 26).