Unknown Calendar: where does it come from?
Looking at the directories:

~/Library/Calendars/*.calendar


where all calendars are stored, the command:

grep string ~/Library/Calendars/*.calendar


let me identify among the 4 strings found, which one of these directories
was the one containing my Calendar ghost calendar:

    [...]
    &lt;string&gt;#0E61B9FF&lt;/string&gt;
    &lt;string&gt;22228888-4444-6666-AFAF-AAFF00005555&lt;/string&gt;
    &lt;string&gt;Calendar&lt;/string&gt;
    &lt;string&gt;Local&lt;/string&gt;
    [...]


From the date of modification of directory:

% ls -l 22228888-4444-6666-AFAF-AAFF00005555.calendar
drwx------  4 hal  hal  170 Sep 21 17:10 22228888-4444-6666-AFAF-AAFF00005555.calendar
%


I diagnosed that this Calendar was imported through my first
synchronisation from a new iPhone containing just its default calendar
named Calendar.

Even with all my calendars more fresh on my Mac, this unexistent default
Calendar was imported.

I removed it on my Mac and made one more synchronisation to get rid of it on my iPhone and my Mac.