We get two free events every month.
One is a new event and the other is an older - re-released - event.
Tomorrow we will have a new event, more info in the Forum News section.
We often say free events are shop-bought events that, when we play them for the first time, we get back more gems than the event cost.
A list of these can be found on the Wiki.
But before you buy any of them, check the event's details page to see if you can play through it.
You can buy these in-game for gems.
APP - You can find them in the map manager.
PC - In the bottom left corner of the Events menu