They can't just say something is a special event. The law is quite clear what is designated a special event:
943.13 (1e) (h) “Special event” means an event that is open to the public, is for a duration of not more than 3 weeks, and either has designated entrances to and from the event that are locked when the event is closed or requires an admission.
So as long as they don't charge and it is an open venue (no locking gate), they can't do anything to prohibit the law abiding from exercising their right to bear arms.
Edit: They also have to post the entrance(s). So even if the event is less than 3 weeks, has locked gates, and requires admission, does not automatically mean it is off limits to carry. It must be posted.