The SelectedDate is not visible because the Template from the DayTemplateSelector is applied on the top of the CalendarButton. However what you can do is to modify the ControlTemplate of the CalendarButton in order to include an additional border which will be used to indicate the SelectedDate by modifying its opacity in the Selected VisualState. For your convenience I have prepared a sample project which demonstrates the exact approach (check the CalendarButtonStyle in the merged resource dictionary).
Hope this helps.
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