The unique building where you can find the chest is marked with a chest icon.
NOTE: The chests appear only if you have enough (more than 3) unplayed match-3 levels in Ship, Cave and Pyramid.
You should reach XP level 12 to participate in this event. If you’re above level 12 and the event hasn’t started for you, you may be playing on an older version of the game. Please update to the most recent version through your device's app store.
The News tab in Settings shows all upcoming events.