Gancio is a free open source Fediverse event planning system, aimed at communities wanting to organise their own events instead of using Facebook or Google.
You can interact with Gancio events from Mastodon etc through ActivityPub, and it also supports RSS.
The Gancio website is at https://gancio.org and the project's official Fediverse account is at @gancio
The software's code is at https://framagit.org/les/gancio