I came across a very promising project called Mobilizon, which will soon launch development. It will be a user-friendly event-managing software program which can be self-hosted. The project is in the funding phase, and there are plans to release a beta version in 2019.
Mobilizon is made by Framasoft, the organization which made PeerTube. Those folks clearly have a great track record.
I want to share this project on our forum just so we keep an eye on it. I get questions about event-managing software at conferences, and I agree that it is one of the most important tools which is missing from the FOSS ecosystem. People rely on Facebook because it offers very usable event-management software. If Mobilizon succeeds, I would love to find a way to integrate it into FreedomBox.