jvalleroy.fbx.one is a Fediverse instance that uses the ActivityPub protocol. In other words, users at this host can communicate with people that use software like Mastodon, Pleroma, Friendica, etc. all around the world.
This server runs the snac software and there is no automatic sign-up process.
But my calendar is self-hosted, as an app within Nextcloud (running on my new FreedomBox from Libre Crafts). So I added my Nextcloud account in the Gnome settings, and it shows appointment reminders that are integrated into the desktop (appear under the clock).