These instructions apply to both Kontact and Korganizer.
KDEPIM 4.6 and later
Starting with KDEPIM 4.6, released with KDE SC 4.6.3, Kontact and Korganizer gained native CalDAV support through Akonadi.
To create a new calendar from Korganizer/Kontact right-click in the calendars list (left pane, under the month view and the events details box by default) and choose "Add Calendar…". In the new window select "DAV groupware resource" and follow the initial wizard. DAViCal is part of the servers supported out of the box and selectable here. If you want to go the manual route then the URL http://calendar.example.com/caldav.php/ should be enough to get access to all of your calendars.
KDEPIM 4.4 and earlier
Make sure you have the `kdepim-kresources` package installed (or you won't be able to use remote calendars). The commands below should work for either korganizer or kontact.
Add a new calendar:
- Kontact (or Korganizer) -> <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"
- Example: 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 must create the calendar using the DAViCal web-based administration interface.
- 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. (This caveat may now be outdated.) Because they use WebDAV, these older versions of Kontact will have read-only access, unless the "readonly_webdav_collections" configuration option is set to true (note that this may permit accidental clobbering of changes to shared calendars if some updates from multiple users happen close to the same time.
Support for CalDAV
There is work to produce a CalDAV KResource for KDE 4.1-4.3 here
I can confirm it is working with the latest version of DAViCal, but at the moment the CalDAV KResource is only able to be compiled on KDE 3.x. Trying to compile it under KDE 4.x will fail under compilation. Using the latest published version of libcaldav (simply replace the tarball in lib with the latest version) will enable support for https thanks to Blindauer Emmanuel in which case Kontact will also be able to use Google's CalDAV extension. Since only KDE 3.x is supported your available distributions should be RHEL, CentOS, Fedora < 12, Debian Lenny and Ubuntu < 9.04.