Dear Genomik,
Hmmm... the time to visit Fiji is dependent upon your tolerance to "cold" and rain. In general, the Fijian winter months are the driest and coldest (June, July, August), but this is only a generality. Rain in Fiji, is like fog in San Francisco. There's no guarantee of a full day of sunshine, but most days have at least a few hours of crystal, beautiful weather. As for temperature, the water dips to 80 degrees Farenheit in June and down to 78 to 76 in July and August. If you want to swim/snorkel in bathwater (83-84 degrees), visit Fiji from December through May.
Above water, it's humid (no surprise, eh?). Air temperatures during Fiji's winter are in the high 70's (really nice) and during Fiji's summer, the temperatures are in the high 80's to low 90's (I've never visited during a Fijian summer - but I've been told it feels like a D.C. summer). Fiji's cyclone season is from December through March. A cyclone, in fact, just passed through Fiji on Monday (category 2).
Some of the best accommodation deals are found between January and March (cyclone season). Airfare deals, though, come out in March to stimulate US summer travel. Too bad there's not consistency.
If you have more questions, just write . . .
Scott