There is already a industry standard in ical/ics - this should be your first feed. Another reason to add this one - regular attendees of a group who are keen can import this straight into their personal calendars and get details of all upcoming events.
After that, I'm less opinionated on what format you should use. https://schema.org/Event will help your SEO so that's a plus.
Do others have comments on what open data format to use?
As someone who has worked on an event aggregator for years, getting good information out of event organisers is very difficult. If they are taking the time to update your site with info, making it so it can be passed on is really helpful. (If they want of course, privacy controls, but in my experience almost all events want more publicity)