Calendar v 6.0 is in applications folder and is 40.8 MB. It was opened 8 days ago and had been open for several days. I think I last used it 3 days ago.
The iCal alias was in the dock and showed open (triangle). Command i showed the linked application as Calendar in the application folder. I clicked on the alias in the dock yet nothing happened. I double clicked the Calendar in the Application folder and nothing happened. When I checked all the windows open there was no Calendar window.
The next time I tried to open Calendar from the iCal alias in the Dock or the Calendar in the Applications folder I got the pop up notification with the Finder faces "The application 'Calendar' can't be opened. -1712"
I'm sorry. I found after posting my question there were other -1712 for the Calendar. I restarted and Calendar opened from the Dock ical alias. Thanks you for your time. I'll try to research better before I post next time.