Baikal

A lightweight CalDAV and CardDAV server, Baikal stores calendars, events, tasks, and contacts and serves them to any standards compliant client over the DAV protocols. A small web admin panel handles users, calendars, and address books, and the data can live in SQLite, MySQL, or PostgreSQL without needing a mail server or chat stack alongside it.

Households and small teams use it to move calendars and contacts off hosted accounts while keeping iOS, macOS, Thunderbird, and DAVx5 clients working unchanged.

Self-hosting it keeps appointment and contact details on infrastructure you control, with no vendor pricing and a trivial database to back up.