I'm not too knowledgeable on things but why would it better to have it during the convention? Don't we want people at the convention?
But yeah we should really not have two things at the same time. Can't one of them change dates?
Do one before the convention and one during or after.
It would be better to have it during the convention because the site of the fairgrounds would serve as a place to sleep. If it was a big event, there would be buses to take people back and forth between the convention and the festival. Mostly, the place to sleep.
After the convention? The festival would or could serve as a base of operations. The plan "what do we do at the convention" is developed, and explained. People are trained, and moved to the convention.