By default, Mozilla Thunderbird doesn't have a calendar section. You'll have to install it thorough an extension. Luckily, there is on created by the Mozilla developer and it's called Lightning calendar. The add-on is in form of a compatible XPI file. Download the XPI, then open Thunderbird and access the Addons page. Drag and drop the XPI into the window and it should be installed. Restart Thunderbird, then you'll have the possibility of using a calendar application inside the email client. The calendar also features various options and settings, cloud integration, etc.
Official website: https://www.mozilla.org/en-US/projects/calendar/
