In JEM there is no better way than to copy the event and alter the info.
I've written a post in the past in the (non public) developers forum on how this could be solved, including a main event (festival) with various sub-events, spread over multiple days.
But it would mean a complete restructure of the code in such a drastic way it would be a new component.
A much easier solution could be to create a 1-n relation between events, where one becomes master, and the others slaves. But it's future, unless more people are willing to code....