Ports for the server are configured via ServerAdmin. The values there default to 8008 (for non-SSL) and 8443 (for SSL). iCal should automatically configure itself with the port number set in ServerAdmin as that information is pushed to the directory from where iCal configures itself.
Note that since the CalDAV protocol is just http, the actual "protocol default" port values are 80 and 443 - but we don't use those in iCalServer because they would clash with the regular Apache web server running on OS X Server.