Thanks for the Safe Mode suggestion. The first thing I did was a restart, but I had not attempted a restart in Safe Mode. Unfortunately the Calendar App still would not open. Info on Safe Mode suggested that if the problem continues in Safe Mode it would be necessary to reinstall OS. The utility value of the Calendar App is not worth starting over by reinstalling Ventura OS, but for future reference would not reinstalling the Mac OS require also reinstalling all other added apps?
Fortunately reinstalling the Mac OS was not necessary. Although I have System Settings set to Auto install software updates, I checked and discovered that there was a later version of Ventura available. Don't know why Auto Install did not work, but updated to Ventura 13.4 and Calendar is working again.
Thanks BDAqua and den.thed for the suggestions.