Taveuni's climate is warm and humid year-round, but the seasons matter. The dry season (May–October) is the best time for most activities: diving conditions are generally excellent, hiking trails are less muddy, and rainfall is lower. July and August are the most popular months and the peak for international visitors.
The wet season (November–April) brings heavy and sustained rain, particularly on the windward (eastern) side of the island. Trails can be very muddy and the Lavena Coastal Walk may be impassable. On the other hand, the wet season is when the tagimaucia flower blooms (October–December), if that is your reason for visiting.
Cyclone season runs from November through April, and Fiji does get hit by cyclones — the worst can cause significant disruption to travel. The diving is less affected by weather than the land activities, since conditions in the Somosomo Strait depend more on current and visibility than on rainfall.