Map from Google - Easter_Island

The climate of Easter Island (Rapa Nui) is subtropical, warm and humid in summer and mild in winter, since the average temperatures ranges from 23 °) in the period from January to March, to 18 °C in the period from July to September (the island is situated in the Southern Hemisphere, where the seasons are reversed to those in Europe or North America).
Easter Island is located 3,600 kilometres west of Chile, it's lost in the Pacific Ocean, just south of the Tropic of Capricorn, and is famous for the moai, the large stone statues which are a sort of watchmen of the island.
The daily temperature range is small, because of the influence of the ocean. Although the maximum temperatures are usually below 30 °C, reaching on average 25/26 °C from January to March, it is sometimes possible that they exceed that value all year round, and at times it's possible that they reach 35 °C from January to April. On the contrary, during winter, sometimes the night temperature may drop slightly below 10 °C from June to August.
Here are the average temperatures.
Average temperatures - Easter Island
Easter IslandJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)212121201817161616161819
Max (°C)252625242221202021212224

The rainfall on Easter Island is quite abundant, around 1,100 millimetres per year, and is well distributed throughout the year, though more abundant in April and May. Sometimes, periods of bad weather with heavy rains may occur during the winter, from June to August.
Here is the average precipitation.
Average precipitation - Easter Island
Easter IslandJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)9080801101201009080807080901070

In Easter Island, the wind blows frequently throughout the year.
The amount of sunshine is quite scarce from May to September, while it is good in summer, from December to March, when there are about 10/11 hours of sunshine per day.
The island is located outside the area in which tropical cyclones form.

Easter Island

The sea is warm enough for swimming from December to April, while it is a bit cool, around 20 °C, in the Southern Hemisphere winter.
Sea temperature - Easter Island
Easter IslandJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)242525242221202020202223

When to go

Notwithstanding the fact that Easter Island has a good climate and can be visited all year round, the best time to visit it is the austral summer, from December to March, which is the warmest and sunniest. The heat is bearable, because it's usually tempered by the breezes. There's no shortage of showers in the afternoon or evening, which are usually brief, but sometimes can be violent.

What to pack

In winter (June to August): clothes for spring and autumn, a sweatshirt or sweater and a jacket for the evening; raincoat or umbrella.
In summer (December to February): light clothing, a sweatshirt for the evening, umbrella or light raincoat.