I needed to synchronize Calendar from my Android phone with Desktop. As I use Thunderbird for mailing on PC, it was natural choice to make it use Calendar too. What a headache! Finding out how to set it up was real adventure! Settings are not simple, and both Google and Lightning changed settings at least twice so ALL instructions are outdated.
So, here is „short“ recipe.
First, go to Google Mail, open Calendar then go to Settings and select specific Calendar that you want to share. List of available Calendars is under My Calendars.
Hover over Calendar name and three dot button will show up. Click on it to pop up options windows. Select „Settings and Sharing“. Find option „Integrate calendar“ in displayed menu on the left. Main frame will scroll down to integration settings.
Look up for Calendar ID. Write down that ID (it is basically account email address).
Then look for „Secret address in iCal format“. If it is there that is good. You have done with settings in Google Calendar.
If „Secret address in iCal format“ is not available you have a problem. It is probable that Calendar sharing is turned off for your account. This usually happens if your account is set under GSuite (it is, actually, turned off by default). You have to contact manager for your GSuite domain and ask him to allow Calendar Sharing.
If you are that admin, great! Here is what you have to do: open Google Admin and then GSuite Admin (click on sandwich menu, then Apps / GSuite). Click on „Sharing settings“ and wait options to show up. Then, set „External sharing options for primary calendars“ to „Share all information, and outsiders can change calendars“. Complicated? Thank to Google.
Now you have to get back to Calendar settings on your account to check if „Secret address in iCal format“ is visible. If yes, that is great. However, it seems it does not show up promptly. You may have to wait some (note necessarily short) time until Google updates something. Keep refreshing page, it should show up eventually.
When, after „Secret address in iCal format“ finally show up, you may proceed to setting Thunderbird.
If you do not have Lightning add on for Thunderbird installed, install it.
Then in Thunderbird go to File / New / Calendar.
Select „On the Network“. In new dialog select CalDAV, and in Location set this path https://apidata.googleusercontent.com/caldav/v2/<your_calendar_id>/events. Instead of <your_calendar_id> put CalendarID you saved before. Click Next.
In new dialog that show up set Calendar name. It might be your email address. Select proper email. Click Next.
New window will pop up. That would be Google asking for You to log in and allow Lightning to access Google API. Log in, follow steps and allow Lightning to access your Google Account.
Click finish in Thunderbird dialog.
If you did all well, Lightning will synchronize with Google and Calendar events will show up.
This method works now, but I am sure that in soon future Google would again change something and render this tutorial unusable. Do not blame me.