When serializing a fullCalendar event with JSON.stringify() you may receive an error message like this one:
TypeError:Converting circular structure to JSON
That happens because each event has a reference to its source, which has a reference to the event itself, effectively creating a circular reference that JSON.stringify cannot manage.
The culprit is the event’s “source” property. The way I deal with this is removing the source property before sending the event to its destination (ajax call, database CRUD function, etc):
delete event.source; saveEvent(event);