Please vote for CalDAV support in Kontact, especially if you have a KDE.org account. Apparently the plan is to push development of support upstream to Akonadi or OpenSync, though neither of these projects' sites mention CalDAV anywhere. Nevertheless, support for those libraries would probably enable CalDAV usage in other open source calendar programs. It might be a good idea to use libcaldav, which is tested against DAViCal.
This applies to both Kontact and Korganizer.
Kontact uses WebDAV, which is supported, but not recommended, by DAViCal (particularly for larger calendars or where calendars are shared with other people).
Nevertheless, Kontact seems to work with DAViCal using the remote calendar function.
- Kontact -> <Right Click> Calendar -> Add... -> Calendar in remote file (in French: "Calendrier dans un fichier distant")
- Use the same URL for "Download from" and "Upload to" : http://''calendar.example.com''/caldav.php/''user''/home
- The calendar must exist in order to use it, of course, or Kontact will send an error (such as "file http://calendar.example.com/caldav.php/user/home does not exist"). You can create it using the web-based administration interface.