calypso software package provides CalDAV/CardDAV server with git backend, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install calypso
calypso is installed in your system.
Make ensure the calypso package were installed using the commands given below,
$ sudo dpkg-query -l | grep calypso *
You will get with calypso package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Web Software |
| Package | calypso |
| Version | 1.5-3 |
| Description | CalDAV/CardDAV server with git backend |
| Sub Section | Web |