Map from Google - Fiji

Fiji, an archipelago in the Pacific Ocean, located north of the Tropic of Capricorn, has a tropical climate, with a hot, humid and rainy season from December to April, and a cooler season from June to October. The latter is relatively dry, especially in the small and flat islands, and in the north-western slopes of the main islands (Viti Levu and Vanua Levu), while in the slopes exposed to the south-east, the trade winds may cause showers and thunderstorms even in this season.
For example, in the capital Suva, located in the south-eastern side of the island of Viti Levu, and therefore exposed to the trade winds, even 2,900 millimetres (114 inches) of rain per year fall, of which more than 270 mm (10.6 in) per month from December to April, when it rains more than one day out of two, however, it still rains on average around every second day in the least rainy months, with a minimum of 120 mm (4.7 in) in July. Therefore, here we can speak of equatorial climate, because there is no dry season. Here is the average precipitation.
Average precipitation - Suva
SuvaJanFebMarAprMayJunJulAugSepOctNovDecYear
SuvaJFMAMJJASONDY
Prec. (mm)2802703603002501701202101902102403102910
Prec.(in)1110.614.211.89.86.74.78.37.58.39.412.2114.6
Days181821191613141516151518198

The daytime temperature in Suva, around 30 °C (86 °F) from January to March, drops to 26/27 °C (79/81 °F) from May to October; even night-time temperatures drop slightly, from 23 °C to 20 °C (73 °F to 68 °F). However, in the coolest months, sometimes at night the temperature can go down to 12/13 °C (54/55 °F), so it can get quite cool.
Here are the average temperatures.
Average temperatures - Suva
SuvaJanFebMarAprMayJunJulAugSepOctNovDec
SuvaJFMAMJJASOND
Min (°C)232323222120202020212122
Max (°C)303030282726262626272829
Min (°F)737373727068686868707072
Max (°F)868686828179797979818284

The sea temperature decreases a bit in the winter months, although it remains pleasantly warm for swimming, as can be seen in the following table.
Sea temperature - Suva
SuvaJanFebMarAprMayJunJulAugSepOctNovDec
SuvaJFMAMJJASOND
Sea (°C)282829282726262525262728
Sea (°F)828284828179797777798182

In Nadi, on the western slope of the same island of Viti Levu, rainfall is much lower, 2,050 mm (80.5 in) per year, and while it rains a lot in the rainy season, from December to April, it rains much less from May to October, when precipitation drops below 100 mm (4 in) per month; the driest month is June, with 55 mm (2.2 in) of rain. The rainy days are fewer as well: 7/8 days per month from July to October, and 10 per month in May and June. Here is the average precipitation.
Average precipitation - Nadi
NadiJanFebMarAprMayJunJulAugSepOctNovDecYear
NadiJFMAMJJASONDY
Prec. (mm)305295420220105105556585601302102055
Prec.(in)1211.616.58.74.14.12.22.63.32.45.18.380.9
Days16161915101077881112139

On the contrary, on the western side the temperature is a little higher, because the winds blow from the inland hills, especially in the coolest months, when the maximum temperature is around 28 °C (82 °F), instead of 26 °C (79 °F).
The amount of sunshine is also higher in the leeward side than in the windward side: about 7/8 hours per day in Nadi, and 5/6 hours in Suva.
These differences between the two sides are also found in the other main island, Vanua Levu. In the smaller islands, especially the flat ones, the difference is smaller, because the trade winds do not find hills upon which they could discharge moisture on one side, so that these islands receive about 1,700/1,800 mm (67/71 in) of rain per year, with a minimum in the coolest months.
An exception is represented by the island of Rotuma, because it is located far to the north, and is very rainy because of the latitude: with its 3,500 mm (138 in) of rain per year, it has an equatorial climate, with a minimum from June to September, when around 200 mm (8 in) of rain per month fall anyway. Here is the average precipitation.
Average precipitation - Rotuma
RotumaJanFebMarAprMayJunJulAugSepOctNovDecYear
RotumaJFMAMJJASONDY
Prec. (mm)3403253302652752202051901902802853053210
Prec.(in)13.412.81310.410.88.78.17.57.51111.212126.4
Days181820191919201918181716221

This island is also warmer, especially in winter, which in fact does not exist, and therefore has stable temperatures throughout the year, as we can see from its average temperatures.
Average temperatures - Rotuma
RotumaJanFebMarAprMayJunJulAugSepOctNovDec
RotumaJFMAMJJASOND
Min (°C)242524242424242424242424
Max (°C)313131313130303030303131
Min (°F)757775757575757575757575
Max (°F)888888888886868686868888

In Rotuma, the temperature of the sea remains high throughout the year as well, as can be seen in the following table.
Sea temperature - Rotuma
RotumaJanFebMarAprMayJunJulAugSepOctNovDec
RotumaJFMAMJJASOND
Sea (°C)292929292928282828282929
Sea (°F)848484848482828282828484

Fiji

Fiji can be affected by tropical cyclones, in the period from November to mid-May (although there was an exception with cyclone Bebe, which passed in the second half of October 1972): one more reason to avoid the muggy and rainy season.

When to go

The best time to visit Fiji is from mid-May to mid-October: it's the most dry, cool and sunny, especially in the north-western slopes of the main islands, which are preferable also because they are more sheltered from the wind.

What to pack

In winter (June to August): light clothes for the day, a scarf for the breeze, a sweatshirt for the evening; light raincoat or umbrella. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
In summer (December to February): light clothes, of natural fibres, sun hat, a scarf for the breeze, a light sweatshirt for the evening; light raincoat or umbrella.
In Rotuma, all year round, light clothing, of natural fibres, sun hat, a scarf for the breeze, a light sweatshirt for the evening, light raincoat or umbrella.