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 daily temperature ranges from 23/24 °C (74/75 °F) in the period from January to March, to 18.5 °C (65 °F) in the period from July to September (the island is situated in the Southern Hemisphere, where the seasons are reversed in comparison with those in Europe or North America).
Easter Island is located 3,600 kilometres (2,200 miles) west of Chile, lost in the Pacific Ocean, a little to the 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 (86 °F), reaching on average 27 °C (81 °F) from January to March, they can sometimes exceed this value all year round, while at times they can reach 35 °C (95 °F) from January to April. On the contrary, during winter, sometimes the night temperature may drop slightly below 10 °C (50 °F) from June to August.
Here are the average temperatures.
Average temperatures - Easter Island
Easter IslandJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)202020191816161516161718
Max (°C)272727252322212122222425
Min (°F)686868666461615961616364
Max (°F)818181777372707072727577

The rainfall on Easter Island is quite abundant, around 1,050 millimetres (42 inches) 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 not so good from April to September, while it is good in summer, from December to March.
Sunshine - Easter Island
Easter IslandJanFebMarAprMayJunJulAugSepOctNovDec
Sun (hours)998665566778

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 quite cool, around 20 °C (68 °F), in the Southern Hemisphere winter.
Sea temperature - Easter Island
Easter IslandJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)242525242221202020202223
Sea (°F)757777757270686868687273

When to go

Notwithstanding the fact that Easter Island has a good climate and can be visited all year round, the best time is the austral summer, from December to March, being the warmest and sunniest. The heat is usually bearable, also because it's tempered by sea 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.