To achieve the desired behaviour you will have to edit the template of the DatePicker. You can easily do this using Expression Blend. Just drop a DatePicker control into the design surface, right-click it and choose from the menu Edit Template->Edit a Copy
. Then Blend will generate all the needed resources. Find the WatermarkTextBlock and remove the Text binding.
Please find attached a sample project in which the above-mentioned steps are performed.
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