Recently I have been engaged in the development of Wordpress plugins and for this reason I wanted to publish a plugin on the official Wordpress site. In the past I created extensions for the CMS "Joomla". For this reason I thought that there is not much difference between the two CMS "Wordpress" and "Joomla". 

But there is already a big difference between developing extensions for Joomla and Wordpress. Extensions for Wordpress are called "plugins". If they should be added to the Wordpress website as "modules", then they are called "widgets". Object oriented programming is used in the PHP classes of Wordpress plugins. JavaScript and CSS files come in a separate folder (often called "assets"). The content of a widget is defined in the function "widget($args, $instance)". This function is added in a PHP class, which must inherit ("extends") from the "WP_WIDGET" class. 

Publishing the plugin on Wordpress.org went smoothly. A folder with the files of the plugin (classes, images and "readme.txt") is created and this folder is then uploaded to the developer page of Wordpress.org. When the plugin has been verified by Wordpress.org, you will get access to a "SVN" repository. There the plugin is always published with the description. All files of the plugin are stored in the folder "trunk". Each plugin has its own page where the images of this page are loaded from the folder "asset" and the text from the file "readme.txt". 

The publishing of new versions of a plugin is also done through the same "SVN" repository. A new folder (with the name of the new version number) is then created in the "tags" folder. But the new files must also be added to the "trunk" folder. Version changes are documented under the sections "Changelog" and "Upgrade Notice" in the file "readme.txt".

The plugin can be downloaded from Wordpress.org. It has been tested until Wordpress version 5.4.2. But the Wordpress plugin is available for all Wordpress versions from 3.0 on. The language of the calendar is set in your  Wordpress settings "Design" -> "Widgets". You can also choose between three different calendar designs ("Violet", "Orange", "Light Green").

Link to the Wordpress plugin:

https://wordpress.org/plugins/responsive-calendar-widget/

Source Code of the Wordpress plugin:

https://github.com/a-dridi/Wordpress-Plugin_Responsive-Calendar-Widget