Silverlight Date picker control will not display error message by default, to display error message we need to create style and in that style we need to add validationErrorElement this will be common for all silverlight controls to display error message.
Unfortunately we cannot find this border in the generic Date picker control. Check the below image.
After lot of work i am able to show error message, by adding a “ValidationErrorElement” in the style and created states “Validation States” to display error message when a error occurred.Check the below image.
Now the error message looks like in the below image
You can download this style from here