The climate of Christmas Island, a territory of Australia located in the Southern Indian Ocean, is tropical, with quite stable temperatures, in fact, highs are around 26/27 °C (79/81 °F) from June to November and around 28/29 °C (82/84 °F) from December to May.
Here are the average temperatures of the largest city, Flying Fish Cove (or The Settlement).
The Settlement - Average temperatures
Min (°C)232323232423232222232323
Max (°C)282828282827262626272728
Min (°F)737373737573737272737373
Max (°F)828282828281797979818182

The main difference in the climate is found in the rainfall, which amounts to 2,100 millimeters (83 inches) per year, and it's much more abundant in the rainy season, from November to June, when the north-western monsoon prevails, while the rains are rarer and shorter from July to October, when the south-east trade winds prevail. The rainiest months are January and February, when about 300/350 mm (12/14 in) of rain fall per month, while the driest months are August and September, when precipitation amounts to about 40/50 mm (1.5/2 in) per month. Here is the average precipitation.
The Settlement - Average precipitation

The north-west monsoon is accompanied by high humidity, which brings low clouds and mists on the inland hills, where a forest grows; it can also be accompanied by strong winds that hit in particular the northern coast, and occasionally by cyclones (see below).

Between October and November, at the beginning of the rainy season, the famous red crab migration occurs on the island: millions of crabs come out from the forest, where they had been hiding in the dry season, and migrate to the beach, to mate and lay their eggs.

Red crabs on Christmas Island

The sea is warm enough to swim in all year round, as you can see in the following table.
Christmas Island - Sea temperature
Temp (°C)282929292928272726272828
Temp (°F)828484848482818179818282

Tropical cyclones

Christmas Island is on the edge of the area where tropical cyclones of the Southern Indian Ocean can form, so it is not directly affected by them in the acme of their intensity, but it's brushed by tropical storms or cyclones in the early stages of their formation. In theory, this can happen from November to mid-May, even though, in the last thirty years, it only happened from November to April, with a peak in December.

Best Time

The best time to visit Christmas Island is from July to October, since it is the least rainy and the sunniest period of the year; in particular, the months of August and September stand out as the driest.

What to pack

All year round, bring light clothes, a scarf for the breeze, a light sweatshirt for the evening, and a light raincoat or umbrella.