Do you enable SIP on your Mac? I have to enable SIP to make it work. Otherwise, it will report error because it can not open Calendar folder, even though I give 777 permissions to all Calendar files (when SIP is disabled). Is this normal? It seems Calendar folder is not under the protection of SIP.

On my first try using this, I got an error because I did not have a diary file.

I fixed this by modifying the top of org-mac-iCal as follows:

(defun org-mac-iCal ()
  "Selects checked calendars in iCal.app and imports them into
the the Emacs diary"
  (interactive)

  ;; kill diary buffers then empty diary files to avoid duplicates
  (setq currentBuffer (buffer-name))
  (setq openBuffers (mapcar (function buffer-name) (buffer-list)))
  (omi-kill-diary-buffer openBuffers)
  (with-temp-buffer
    (when (file-exists-p diary-file) ; <------ added this check
     (insert-file-contents diary-file))
    (delete-region (point-min) (point-max))
    (write-region (point-min) (point-max) diary-file))

Hope that's helpful. Seemed too trivial to bother with a full clone and pull request...

I am on 10.14.6 and note the comment about the "Checked" flag in info.plist.

As a work around I have changed in omi-checked
(checked (car (cddr (lax-plist-get keys '(key nil "Checked")))))
to

(checked (car (cddr (lax-plist-get keys '(key nil "Enabled")))))

and included Exchange calendars too. I believe the variables all-calendars or usedCalendarFiles etc hold the correct lists, etc, and read & write iCal files ok. But the diary file output is always empty.

As a work around, I tried to customize org-mac-iCal-calendar-names, but am unsure what you mean by "Title"? Is it the title I see in the Calendar App, do I need to give its type (On My Mac / Work or iCloud Work), or caldav like 8949A8EB-4FF7-43D4-982E-1F116CD21C8F.caldav or the calendar title within the caldav 320B3CAD-E2F7-414D-BE89-118CA4C648CB.calendar? I tested possible combinations but still have empty diary.

I'd appreciate one caldav and one exchange example. I will also experiment on 10.13 to see how this works there. Thanks for the help, I don't know much about the Calendar formats on Mac, hope what I am asking makes sense.