This from Macfixit:
Reimport affected calendars In iCal, select the problematic calendar and choose "Export" from the "File" menu. This will allow the calendar to be saved to a temporary location such as the Desktop. When the calendar has been saved, delete it from iCal and then from the "File" menu, select "import" and import the previously saved calendar. This should allow the calendar to be readable again.
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P
Some of my calendars are read only. How can I change them to "read and write"?

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