1. Go to iCal > Quit iCal
2. Remove the following to the trash and restart your computer:
Home > Library > Caches > com.apple.ical
Home > Library > Calendars
Home > Library > Preferences > com.apple.ical (There may be more than one of these. Remove them all.)
--- NOTE: To get to "Home > Library" in Lion: Click anywhere on the desktop and then click on the "Go" menu at the top of the computer screen while holding down the "option" key on the keyboard. You will see "Library" in the menu.
--- NOTE: Removing these files may remove any shared (CalDAV) calendars you may have access to. You will have to re-add those calendars to iCal > Preferences > Accounts.
3. Launch iCal and test.
Hope that helps.