This can happen if you are using iCloud and have the option for Calendars checked in iCloud preferences ( Apple > System Preferences > iCloud).
If that isn't it:
1. Make an iCal backup: Click on each calendar on the left hand side of iCal one at a time highlighting it's name and then going to File Export > Export and saving the resulting calendar file to a logical location for safekeeping.
2. Go to iCal > Quit iCal
3. Remove the following to the trash and restart your computer:
Home > Library > Caches > com.apple.ical (There may be more than one of these. Remove all files and folders that start with this phrase)
Home > Library > Calendars > Calendar Cache, Cache, Cache 1, 2, 3, etc. (Do not remove Sync Cache or Theme Cache if present)
Home > Library > Preferences > com.apple.ical (There may be more than one of these. Remove all files and folders that start with this phrase.)
Home > Library > Preferences > ByHost >com.apple.ical (There may be more than one of these. Remove all files and folders that start with this phrase.)
--- 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.
--- 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.
Hope that helps.