Radicale

Radicale is a lightweight server that speaks CalDAV and CardDAV, the standards that let calendars and address books sync between devices. It handles events, todos, journals and contacts, works with any compliant client on phones, desktops and webmail, and offers per-user accounts with rights and sharing so a family or team can share one calendar.

People use it to keep a single source of truth for schedules and contacts without handing their data to a provider, and it drops into an existing setup with little fuss.

Running it yourself means the calendar data stays on your disk, there is no per-user fee, and the storage folder is plain enough to back up and restore by hand.