Radicale is a small, free CalDAV and CardDAV server for sharing calendars, to-do lists and contacts. It handles events, to-dos, journal entries and contact cards and works with many CalDAV and CardDAV clients. Radicale needs little configuration, can restrict access through authentication, supports TLS and can be extended with plugins. Data is stored as iCalendar and vCard files in a simple folder structure on the file system. Radicale is only the server, not a client. You install it yourself through pip, a Docker image or a Linux package, optionally behind a reverse proxy such as nginx, Caddy or Apache.
Radicale is free software under the GPLv3 and is developed by the Kozea community; its GitHub repository sits under the Kozea organisation. No service provider hosts your data: you run the server yourself, so calendars and contacts stay on infrastructure of your choice, for example in the EU. Operations, updates and security remain your own responsibility.