Calendars, contacts, and more can be synchronized using the Funambol sync server with a Citadel module written by Mathew McBride. You can learn more about this solution here:
There is an Android app called ICSsync which does a great job of synchronizing your mobile device's calendar with your Citadel calendar. Simply install ICSsync on your Android device and then click “Add Feed.”
Your feed URL should be set to: http://your.citadel.server/groupdav/calendar/calendar.ics
Your username and password should be set to the same as you use to log in to Citadel from your computer.
After you complete your first sync, your Citadel calendar will appear on your Android device, and you will be able to view, create, and edit events. That's all there is to it!