I want to suggest adding the function of marking some days as important or special in the calendar.
I was faced with such a need when I was tasked with writing a production calendar.
But the application for this function can be much more.
For example: creating schedules, todo lists, specific calendars, such as for example: the calendar of the female cycle.
As a functional, I would suggest the following:
add property: types of important days - an array of properties, such as a brush.
And add a method: "Add an important day" (AddImportantDay(DateTime d, int index) )
As parameters in it, take a date and an index of the type of an important day.