Radicale

Radicale is a small but powerful CalDAV and CardDAV server written in Python. It lets you share and synchronize calendars, to-do lists, journal entries, and contacts across your devices using the standard CalDAV and CardDAV protocols, giving you a self-hosted alternative to cloud calendar and contact services.

It works out of the box with minimal configuration, storing all data on the filesystem in a simple folder structure that is easy to back up and inspect. It supports flexible authentication and per-collection access control, TLS encryption, and an extensible plugin system for custom storage, authentication, and rights backends.

Typical use is as a private, low-footprint sync server for personal or small-team calendars and address books, working with a broad range of clients such as Thunderbird, Evolution, DAVx5 on Android, and the built-in calendar/contacts apps on iOS and macOS.