Mobilizon is an event organising platform for the Fediverse, a free open source alternative to Facebook Events.
You can find out more at:
Mobilizon instances federate with the rest of the Fediverse, so people on Mastodon can follow Mobilizon accounts and see their latest events.
You can join an existing instance, there's a list of instances at https://instances.joinmobilizon.org
If you're technically skilled, you can create your own instance using the instructions at https://docs.joinmobilizon.org/administration/install