Description:

Vdirsyncer is a command-line tool for synchronizing calendars and
addressbooks between a variety of servers and the local filesystem.
The most popular usecase is to synchronize a server with a local
folder and use a set of other programs to change the local events and
contacts. Vdirsyncer can then synchronize those changes back to the
server.

However, vdirsyncer is not limited to synchronizing between clients
and servers. It can also be used to synchronize calendars and/or
addressbooks between two servers directly.