Dry Season: June - September
Rainy Season: November - April
The austral winter, from June to September brings the cooler and more comfortable dry season to Fiji. Travelers enjoy clear waters for water sports and cool hiking opportunities. Fiji offers some of the finest dives of the South Pacific, and the dry season with great visibility and 79 degree water temperature is ideal.
If you do not mind the humidity and a few showers, the wet season, from November to April, may grant you a better bargain. Humidity is high though, showers can sometimes last for days, and tropical storms are more likely from December to mid-April. If crowd avoidance is high in your priorities, then November, February and March are your best bet.
Another factor to consider is what part of the island you visit. Since the moist trade winds blow from the east, places on the western side, like Nadi and the Mamacuna Islands tend to be dryer than places on the east side like the Coral Coast  and Suva. Also keep in mind when hiking or visiting villages on the mountains that places with higher elevation are cooler than at sea level.
Local holidays and festivities bring people out to tourist attractions, and crowd resorts and beaches. Expect lots of Australians, New Zealanders, and Fijians during December and January. Fijian school holidays also become remarkably busy times from April to mid-May, mid-August to early September and December to late January. The Diwali Festival in October and early November is also particularly active.