Symptoms

If you don't immediately migrate your iCal data in Migration Assistant after installing Mac OS X v10.5 Leopard or later, and you open and close Mail or the Calendar widget before opening iCal (even if you use Migration Assistant after opening Mail or the Calendar widget), empty "Home" and "Work" calendars may appear.

Resolution

Use one of these solutions to restore your calendar data.

Manually copy calendar data

If you copied your calendar data manually to "Users/~/Library/Application Support/iCal" but only the default "Home" and "Work" calendars appear when you open iCal, quit all open applications (including Mail and iCal) and move the "Users/~/Library/Calendars" folder to the desktop. Open iCal again.

If you created an iCal Backup File

Double-click your iCal Backup File (this is the file that's created when choosing Backup Database from the File menu in iCal). This will restore your calendar data.

When you next open iCal, your previous calendar data should be restored.