If you use WP Optimize plugin, and you have issue of not showing calendar, but instead of that see the "Calendar loading...".
Probably in the Developer console you will see several lines such as this:

== WPBC VARS 10.1.2 [free] LOADED == wpo-minify-footer-4d836f9d.js:3429:3326
== WPBC VARS 10.1.2 [free] LOADED == wpo-minify-footer-4d836f9d.js:5426:3326
== WPBC VARS 10.1.2 [free] LOADED == wpo-minify-footer-4d836f9d.js:7948:3326
...

Here's how to resolve these issues:

1. Update Your Booking Calendar to version 10.1.3 or newer
   Ensure you have updated your Booking Calendar to version 10.1.3 or newer. This update addresses cache plugin issues.
   Important! After updating, clear all minimized CSS and JavaScript files and cached pages or posts.

   If you have a paid version, you can request the latest update on this page.

2. For Booking Calendar Versions 10.1.2 or Older

   1) Please open the WP Optimize > Minify > JavaScript menu and these rules to "Exclude JavaScript from processing":
   

   */plugins/booking*
   */jquery/jquery.min.js
   */jquery/jquery-migrate.min.js
   

   
   

   2) If by some reason, you do not want to exclude those files, then please check these steps. But we reccomend todo steps from point #1.
   In this case you need to open the WP Optimize > Minify > JavaScript menu
   and deactivate this option: "Enable merging of JavaScript files".
   Save changes.
   Test it.