iCal Files iCal keeps its files in the Library folder in your home directory, the calendars as .ics files in the Library/Calendars folder, and the preferences as .plist files in the Library/Preferences folder. There are 4 or 5 preference files, all with names beginning with com.apple.iCal.
Testing iCal If you are experiencing problems, a good way to start diagnosis is to create a new user account, log on as the new user, and run iCal. As iCal maintains separate files for each user, if the problems recur with the new user it is unlikely to be a preference or calendar file issue, and you should do things like repairing permissions, looking for faulty fonts, checking disk capacity etc.
Deleting Preference Files If iCal behaves in the new user account, switch back to your normal account. Start iCal and make a note of its preference settings. Quit iCal and delete its preference files from ~/Library/Preferences. Restart iCal, reset the preferences, and try it out.
Testing Calendar Files If the problems are still there, uncheck all the calendars in the calendar list, and make a new calendar. If the problems persist with the new calendar, I haven't a clue what the problem is, so post a message in the discussions and see if anyone else can help. If the misbehaviour is now gone, you need to find which of your calendars is bad.
Finding Bad Calendar Files Turn back on the old calendars one at a time. One of them should start the problems again, and is presumably damaged. Quit iCal, and drag the damaged file from ~/Library/Calendars to the desktop, and restart iCal. Note that the calendar you removed is still listed, but empty. iCal keeps the calendar names in a preference file, and makes a new empty file for a missing calendar.
Recovering Events You can try to recover events from the desktop calendar file by using import. If there is a lot of data in the damaged file, and import won't recover it, you can read the file with Text Edit or other text editor, and at worst see what is missing. You may even be able to spot the damage, and cut it out - but if trying this, work on a copy, just in case ... If you think you have repaired the damage, with iCal shut down you can pop the ics file back where it came from, replacing the empty one that is in its place.
iCal Won't Start If iCal won't start properly, test the preferences as above, but you won't be able to uncheck the calendar display, so drag all the calendar files to the desktop and see if that helps. If it now starts you can put back the calendars one at a time, with iCal shut down when you move one of its files. If it won't start with its calendars moved and its preferences deleted another possibility is a bad Address Book data base. Drag (with Address Book shut down) the AddressBook folder from ~/Library/Application Support to the desktop and give iCal another try.
Backups iCal is fragile. Your data is important. Back it up daily.
Original tip by Austin Kinsella.