Adding Calendar Support to Thunderbird

Click Tools then Add-ons
Type lightning in the search box (1), then click the magnifying glass icon (2)
When the search completes, click Add to Thunderbird on the Lightning extension
Wait for the countdown to complete and click Install Now
Once the install completes, click Restart Thunderbird
You now have 1)Events and Tasks button, 2)Direct Calendar button, and 3)Day Events pane