Use this plugin with widget()

I’m really not sure what you mean. My Calendar has several widgets that can be used if your theme is enabled for widgets. Is that what you mean?

Hello
My wordpress theme has no widget in the header. I would change “header.php” to add a mini calendar. Do you have a solution?

Well, you have two choices: add a widget in the header, or use the shortcode and run it using do_shortcode().

https://codex.www.ads-software.com/Widgetizing_Themes

https://codex.www.ads-software.com/Function_Reference/do_shortcode

Ok thank you very much. I will look at it ! ??