So I had to restrict days in the WPF DatePicker component. Only the days falling within a specific range should be available for selection and all others (past & future values) should be disabled.

The solution is to use the DisplayDateStart & DisplayDateEnd properties on the DatePicker

<DatePicker Name="dtPicker"
    DisplayDateStart="5/10/2012"
    DisplayDateEnd="5/20/2012" />