French Polynesia has two seasons. The dry season runs roughly from May to October: lower humidity, temperatures in the mid-20s to low 30s Celsius, little rain, and reliable sunshine. This is the most comfortable time to visit and also the busiest and most expensive. July is peak season — the Heiva festival draws visitors from across the islands, and prices for accommodation climb accordingly.
The wet season runs from November to April: hotter, more humid, with frequent tropical showers — though these are usually short and followed by sunshine. Cyclones are rare but possible, mostly January through March. Accommodation is cheaper and the islands are quieter. If you don't mind the occasional downpour, this can be a good time to visit.
May and September are the sweet spots: good weather, lower prices, fewer crowds than the July peak. September is also the best month for whale watching — humpbacks pass through on their migration and are often visible from shore or on short boat excursions.
The Marquesas have a slightly different climate from the Society Islands — they are less dependent on seasonal variation and can be visited year-round, though the wet season (December to March) brings heavy rain to the valleys.