This plugin is a Calendar widget, it was customized from the WordPress Calendar Widget.
With this widget, it will show list of posts with link on day that mouse hover to it.
Its setting allows to choose the category to query and can choose the color for background and font that will show on.
Once installed and enabled it, go to widget setting and find for widget “Calendar with posts”, drag it to where you want it to be, then if you want just give it a title, choose a category, background color, font color.