Sure! If not, it's kinda hard to use MB as an event calendar, which is part of the idea. In fact, we'd love to have all the data in advance!
We can change them.
That's why we have things like "Cancelled" checkboxes for events. Even if they do not happen, we'd like to know it was supposed to happen but didn't - that's also interesting data.
Maybe they work for the festival. Maybe they just feel like it. Maybe they run some sort of event site using MusicBrainz data, which is perfectly legitimate (as long as we know about it, anyway).