Force Quit Calendar, from the menu by choosing >Force Quit...>Calendar.
Paste this text ~/Library into the Finder Menu>Go>Go to Folder...window and select "Go." This will open the Users Library Folder.
Then open the Calendars Folder which is in that Library Folder, and move any any files with "Cache" listed in the name field to your Desktop.
Next open the Preferences Folder in that same Library Folder and move the com.apple.iCal.plist file to the Desktop.
Log out/in or restart and open Calendar.