When flying from Johannesburg to Easter Island, the time of day you decide to fly out makes little to no difference towards the overall price you’ll pay on tickets.
When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Johannesburg is roughly similar to Easter Island. You can assume this to be the case over the next 15 days.
You can imagine temperatures to be in the 28 °C range in February. The weather in Easter Island is generally hotter than Johannesburg's. As flight availability is subject to become more limited as peak season approaches it is ideal to book in a timely manner.