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!
In response to community demand, support for CalDAV and CardDAV are currently being developed for the Citadel system.