Add Google Calendar to Thunderbird Mail Client

There are two plugins that must be installed:

• Lightning – the actual calendar plugin
• Provider for Google – allows bidirectional access to Google Calendar

Install both of these plugins, from within Thunderbird (Menu button | Plugins | See all and then search for Lightning and then Provider for Google. Click Add to Thunderbird and then, when prompted, restart the app).

Once these have been installed, and Thunderbird restarted, you’re ready to configure.

Adding a Google Calendar
Open up the Lightning calendar tab (click on the Lightning icon in the top right corner of the Thunderbird window). Right-click a blank spot in the Calendar pane (left side of the window) and click New Calendar. In the resulting window (Figure A), click On the Network and then click Next.

Figure A

Creating a new calendar in Lightning.
In the Locate your calendar window (Figure B), select Google Calendar, and click Next.

Figure B

The Google Calendar is now an option, thanks to Provider for Google.
You will then be prompted to enter the email address associated with the Google account that contains the calendar to be added. Enter that and type Next. The resulting window is the standard Google signin screen, where you’ll enter your email address, password, and (if applicable—and it should be applicable—because you should be using two-factor authentication), your two-factor authentication code. Finish that signin process and then click ALLOW to give Provider for Google access to your calendars and tasks. Once you click ALLOW, you will be taken back to the Create New Calendar wizard, where you can then select the calendars (associated with that Google account) you want to include (Figure C).

Figure C

Selecting your calendars.
After you’ve selected your calendars, Click Next and then Finish. You’re done. The necessary Google Calendars have been added with both read and write access from within Thunderbird.

Src: https://www.techrepublic.com/article/how-to-add-a-readwrite-google-calendar-to-thunderbirds-lightning/