Error message from calendar program
I just got off the phone with support and now it's fixed. This is what we did:


System Preferences - > Internet Accounts
Select the iCloud account. 
Uncheck "Calendars"
Open a Finder Window. 
Go to the ~/Library folder. (He had me click on the Go menu and press the Option key in order to display the Library and then go to it from there.)
Copy ~/Library/Calendars onto the desktop, then delete the on the one in the ~/Library folder. 
Restart. 
System Preferences - > Internet Accounts
Select the iCloud account. 
Check "Calendars"
Open the Calendars app. Wait for all the calendars and events to reload from the cloud. 


That's it. Test by trying to make or edit events. If all is well, delete the back-up.