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