You could easily customize the appearance of the day button by setting the DayTemplateSelector
of the RadCalendar. You could find an example illustrating this in our online demos: http://demos.telerik.com/wpf/ RadCalendar->First Look
Also, you could refer to this help article for more information: http://www.telerik.com/help/wpf/radcalendar-styling-and-appearance-daybuttonstyleselector.html
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