Q: Retrieving calendar info after installing Mavericks

If the appointment is no longer on your calendar you would need a backup source to recover it from, such as a time machine backup.

I'm not sure if the folder from pre-Mavericks would be compatible.  You would probably have to ask over in the Mavericks forum: https://discussions.apple.com/community/mac_os/os_x_mavericks?view=discussions.  If you have a Time Machine backup, you could restore your iCloud calendars by following Roger Wilmut's article here: http://www.wilmut.webspace.virginmedia.com/notes/icloudtm.html.

As far as I know, you restore the entire Calendars folder (located at ~/Library/Calendars).  You could ask over in the Mavericks forum to confirm this.