After all you find on the web is only half the truth, here's the real deal:
Go to
/Users/<youruser>/Library/Calendars/...
(that much is true (iWolfgang - thanks!))
Find out which of the xxx.caldav-Folders it the one of your iCloud-calendar (if you have multiple calendars like me):
The contained "Info.plist"-file should contain something like this:
(if it contains something like "group.calendar.google.com" you're in the wrong folder ;-) )
Found the right folder? Fine!
So this is the URL to start (not ready yet!):
https://p0X-caldav.icloud.com/1234567890/
(The "X" should be a number, and "1234567890" should be some different ID (your ID))
Now check the sub-folders of the main-folder you found. Open the "Info.plist"-file again. You will find something like this:
<key>CalendarPath</key> |
<string>/1234567890/calendars/work/</string> |
(Example for your "work" folder)
Combine the infos you found to this URL:
https://p0X-caldav.icloud.com/1234567890/calendars/work/
This is the URL you got to use with Lightning (New calendar -> network -> choose "CalDAV" / address: see above)!
Enjoy!
Stephan