The dry season, May to October, is the best time to visit. Temperatures are cooler (20–25°C), humidity is lower, and the sea is calmer — better for diving, snorkelling, and inter-island travel. This also coincides with whale season.
Whale season runs from late June to early November, with July, August, and September being the prime months. Humpback whales migrate from Antarctic waters to breed and nurse their calves in the warm, sheltered waters around Vava'u, Ha'apai, and Tongatapu. Tonga is one of the very few places in the world where swimming with humpback whales is permitted, under strict regulations. Tours from licensed operators in Vava'u and Ha'apai fill quickly — book months in advance if this is your main reason for coming.
The wet season, November to April, brings heat, humidity, and cyclone risk. January through March is the highest-risk period. Cyclones are not inevitable but they do hit: Cyclone Harold in 2020 and Cyclone Gita in 2018 both caused significant damage. Some accommodation on outer islands closes in these months, and ferry and flight schedules become even less reliable. On the plus side, prices are lower and you'll have more places to yourself.
The Heilala Festival in early July — centred on the King's Birthday — is Tonga's biggest cultural event. It brings parades, brass bands, traditional dance, and the Miss Heilala pageant to Nuku'alofa. If you're in Tonga in early July, it's worth timing your visit to Tongatapu around it.