iCal Server is a service included in Mac OS X Server v10.5. iCal Server is based on the CalDAV calendar standard. The iCal Server is a CalDAV standards-compliant server that allows multiple users to collaboratively share calendaring information.
iCal Server was announced during Apple's Worldwide Developers Conference on August 7, 2006. It uses an open-source webserver, Darwin Calendar Server, which was initiated by Apple at the same time, and has the flexibility to run on different platforms without the iCal Server GUI.
iCal Server was the first proprietary calendar server to support the open CalDAV standard. Apple has released an opensource calendar server
iCal Server is based on on the HTTP, WebDAV, iCalendar and iTIP standards.