Hi Joe and welcome back. 🙂
Good question. When the default-template.php file is added to the theme, then it will assume your custom template is what you want to use for the calendar instead of one of your theme’s page templates.
You can get the sidebar into the template by adding the sidebar code for it in the template. If you open up one of your theme’s page templates (e.g. page.php) then that likely will contain the code you are looking for and you can add it to the calendar template where you would like it to appear.
Will this help you get started? Please let me know.
Cheers!
Geoff