To reinstall iCal on Mac OS X Lion and Mac OS X Mountain Lion, you will need to reinstall the OS X:
To reinstall an app that came with your Mac, such as Mail or Calendar, reinstall OS X. Reinstalling OS X doesn’t erase your information.
For more information about reinstalling OS X, see this help topic:
You can find the full article here (the article title says OS X Mountain Lion, but this is true for OS X Lion as well):
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