Icon in the apps menu launches the settings screen (the number on the icon is not updated). For app work you need to add the widget itself from widgets menu to the home screen.
In the settings you can set the font size for the date, day of week and month.
If the name of the month cut or is missing - you need to reduce the font for the date. This is due to the fact that in different models of smartphones system fonts are different.
Also you can choose title color for workdays and weekends, and to mark what days of the week to count the weekends.
Click on the widget starts the system Calendar app, or a selection of other calendars, if installed.