Calendar broken after upgrade to High Sierra
Try this: 
Quit calendar (cmd + q), delete the calendar directory on your Mac’s HD/SSD, empty trash, reboot (with the option to open previous apps/Windows off), launch calendar, wait 5 minutes, quit calendar (cmd + q). Find the .ics files in the backup, import them with the calendar app, wait, see if your calendar works.

Another soloution may be:
Quit calendar
Find the old calendar app in your backup, and copy it to your desktop.
Copy the calendar directory from your backup, replace the current directory on your Mac. Reboot (with the option to open previous apps/Windows off).
Launch calendar from your desktop, and see if it works...