Because of the complexity of iCal's integration with applications like Mail and Address Book, not to mention syncing with MobileMe, users may find iCal not functioning as they had hoped. Apple Support Discussions user "jabarrett" reports:
All of my calendars have disappeared. can't post or edit events, etc. Just spent the better part of 2 hours with Mac support person - no luck in fixing the problem.ASD user "kyla.m" adds:
The small calendar is also stuck on January 2001, and I cannot add anything to the calendar at all. I tried syncing it with my iPhone, and everything else synced except the iCal. I tried importing the contents of another iCal, and it froze - I can't even close it without forcing it to quit.
Often, iCal can experience freezes because of an issue when attempting to sync with MobileMe. Try turning iCal sync off. Navigate to System Preferences > MobileMe > Sync and deselect "Calendars". You can also open iCal, navigate to iCal > Preferences > General and deselect "Synchronize iCal with other computers and devices using MoblieMe".
Calendar Cache and Preference Files
To test the Calendar Cache file, navigate to (username > Library > Calendars) and remove the Calendar Cache file. If you find you have more than one Calendar Cache file, remove all of them.
As with many Apple applications, removing the iCal .plist file from its location (username > Library > Preferences) and restarting the application can solve many issues. With iCal, you will want to test both the (com.apple.iCal.plist) and the (com.apple.iCal.helper.plist) files. Simply drag them to the Desktop and see if you issue still exists after starting iCal again. Note: Removing the .plist files will result in lost information about your calendars.
Mac OS X Combo Update
Since iCal is integrated into Mac OS X, reinstalling the latest Combo Update for your version of the OS can often result in issues clearing up. Currently, there is no Combo Update available for users of 10.6 Snow Leopard. Users with Leopard (10.5.8) can download the last Combo Update here.
A more tedious approach to troubleshooting iCal issues involves inspecting individual calendars. Be sure the dates and times entered for each event are correct and do not contain foreign characters (stick to letters and numbers). If you are syncing your birthdays from Address Book to iCal, be sure all the birthdays listed in Address Book have correct characters entered as well. One corrupt event can cause the entire calendar (or the entire application) to act up.