Weather
Fiji is a tropical country with two seasons: wet (rainy) and dry. December is at the beginning of the wet season, which starts in November and continues until April. This season is also sometimes called Fiji's summer. While rainfall levels aren't as high in December as between January and March, you should still expect some rain every day. Being "summer," December is also hot and humid. When it's not raining, the sun can be scorching. Although temperatures are fairly uniform year-round, expect high temperatures of up to 84°F (29°C) in December and warm nights.
Although December is outside the typical cyclone season (January to March), there's still a small threat from early cyclones. These bring very heavy rains and wind and can be very destructive. There's usually plenty of warning when a cyclone is on its way to Fiji, so keep plans flexible and be prepared to shift course if you end up being in a cyclone's path in December.
Crowds & Costs
While the wet season is generally the low season for travel to Fiji, the major exception is in the second half of the month, around Christmas and New Year. This is one of the most expensive times to visit the country, and you should make bookings well in advance. Despite the chance of rain, Australians and New Zealanders flock to Fiji over their Christmas holidays, filling up resorts. If you have flexibility in your plans, consider visiting earlier in December, before Antipodean schools are out for summer.
Where to Go
Choose your beach location in Fiji wisely during the rainy season so you don't get wetter than you need to. For good reason, many of Viti Levu's resorts are clustered around the west and southwest coast: this is the drier side of the island. It's also convenient if you've flown into Nadi. For luxurious five-star treatment, head to Denarau Island, an enclave built on reclaimed mangrove swamps that's home to a number of high-end resorts and private beaches.
Another advantage of staying around Nadi or Denarau is that this is where boats transfer to the lovely Yasawa Islands. There are 12 main islands in the Yasawa Group and ferry services between them, making it easy to island hop. Don't miss visiting the Sawa-I-Lau Caves on Yasawa Island. These are most easily reached from resorts on Yasawa, which often offer day trip options. The caves are inside a mountain and are fed by seawater. Swim in them to cool off from the rainy season heat.
What to Do
Consider staying in a homestay in Fiji instead of (or as well as) a fancy resort. If you're visiting later in December, then this is an especially good idea because you can see how Fijians typically celebrate Christmas and New Year (the latter is traditionally a week-long celebration). Homestays in villages are a great way to interact with local people you wouldn't otherwise meet and to learn about indigenous Fijian culture. Stays usually include a village tour and a kava ceremony.
An island-hopping itinerary is ideal if you prefer seeing new places each day rather than staying put in one place. Various destinations in Fiji lend themselves well to island-hopping trips, whether sleeping on dry land or a luxurious charter yacht. The 20 islands of the Mamanuca Group offer idyllic white-sand beaches, while the volcanic atolls of Yasawa Group are equally gorgeous and a little more rugged.
Events in December
Christmas, nationwide. Many Fijians are Christians and Christmas Day (December 25) and Boxing Day (December 26) are public holidays. If you're staying in a resort, you might be able to enjoy a special Christmas meal.
New Year's Eve, nationwide. The New Year is traditionally celebrated over a week-long period in Fiji. Expect big parties with live music, fireworks in Suva and Nadi, and special resort events. To participate in a traditional New Year's celebration, stay in a homestay in the lead up to December 31. January 1 is a public holiday in Fiji.