Thunderbird is not just an app to read and send emails. You can also use it to manage your schedules and tasks. The calendar feature of Thunderbird — which allows you to manage schedules — supports iCalendar. Thanks to this, you can view calendars from other calendar apps that support iCalendar as well, including Google Calendar. iCalendar itself is an open standard format to exchange calendar information between internet-based calendar apps. The iCalendar files typically have the extension of .ical, .ics, .ifb, or .icalendar.
To transfer a calendar from Google Calendar to Thunderbird, you need the URL that refers to the calendar you want to transfer. To get this URL, you can go to Google Calendar and login with your Google account. Once logged in, hover your mouse over a calendar you want to share on the left panel. Click the three-dot icon and select Settings and sharing.
On the Calendar settings page, scroll down to the Secret address in iCal format section and copy the URL.
As you can see, the URL has the suffix of .ics, indicating that it is an iCalendar file. Once the URL is copied, launch Thunderbird and go to the calendar tab by clicking the Events and Tasks menu and select Calendar (Ctrl+Shift+C).
Create a new calendar by right-clicking on an empty space on the calendar panel (on the left side) and select New Calendar.
On the next step, select From Network and click the Next button.
Paste the iCalendar URL you copied from Google Calendar on the Location field. On the Format option, make sure to select iCalendar (ICS). Click the Next button.
Lastly, give your new calendar a name and click the Next button. Before clicking this button, you can also set the color to make your calendar looks more organized. There is also an option to hide the reminders. On the bottom option, you can select an existing email address on your Thunderbird app where you want to transfer the calendar to (if you have multiple email addresses).
To view the calendar you have just imported from Google Calendar, you just need to make sure that the imported calendar is checked on the left panel. You can use the navigation panel to jump between days, weeks, or months according to the view mode you are on.
Please note that importing a Google Calendar to Thunderbird doesn’t allow you to manage the imported calendar from the Thunderbird window. You won’t be able to add a new event or delete the existing ones. Even so, all the changes you made on the Google Calendar app will be applied to Thunderbird. You can click the Syncronize button to manually sync your Google Calendar with Thunderbird.
If you want to stop displaying your Google Calendar in Thunderbird, you can simply right-click the calendar on the left panel and select Unsubscribe Calendar.