1) Try removing the com.apple.iCal.plist file from your Macintosh HD/Users/yourusername/Library/Preferences Folder. To find that Library folder if you are using Lion/Mountain Lion, use Finder>Go Menu>Depress the "Option" key>Library. Drag the .plist file to your desktop, and log out/in or restart.
2) Next, make sure that you are not connected to the internet. Then go to your Macintosh HD/Users/yourusername/Library/Calendars Folder and remove any files with "Cache" as a part of the file name. Use the same method as listed above to get to the correct Library Folder. Log out/in, or restart and check iCal for functionality.
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https://discussions.apple.com/message/20069720