Event chests - are the chests, that appear on your island every day randomly if you are on the 15 level or higher. Usually, they come with the time limited event and appear till the end of the event.  

These chests can be found on the land or on water. You can open them by preparing the needed resources and after that, you can claim the reward or leave the chest as a decoration. 

Be careful to prepare the ingredients in time, because the time to open the chest is limited.

As the place of their spawn is random, sometimes it can be challenging to find it if you have a lot of buildings and decorations on your island or on water.