For some reason this is not that intuitive the first time and there don’t seem to be many/any specific tutorials out there. I kept getting a ‘modification fail’ error message or other errors. So, here you go:
In Nextcloud/Owncloud
1. Log in to your browser-based owncloud/nextcloud page
2. Go to the top left and click the down arrow to access the calendar app
Note: this *must* be enabled first by your admin, if you happen also to be your own admin
3. Grab the caldav link from the … share icon drop down
4. Select the content of the link and copy it to your clipboard (control A/Control C)
Back in Thunderbird Lightning
Note: You must first have the Lightning add-on installed in Thunderbird if yours does not already have it. It should come default but I recall in the past it did not…
Now skip past step #10 in this tutorial, and start at the ‘Back in Thunderbird Lightning’ steps
The key point is that it is ‘caldav’ that you select, *not* ical.
This was very helpful. I couldn’t get the iCal to work for me in Nextcloud 10.0.4, but this CalDav method worked perfectly on Thunderbird 52.
Thunderbird needs to take advantage of owncloud/nextcloud account info so that calendars get setup automatically instead of individually. Linux does this for Gnome and it makes it VERY simple.