I am not sure. The basic idea is that you would put the label of the field in the message. That way the user would see a message saying "Begin Date is not a valid date". This seems like it would be pretty easy if you implemented the validate method yourself in Java code, but I don't know how to use a dynamic value using the validation framework.
BTW, it may not even be a useful thing to do. When a validation error triggers, the user can look at the value in the "Begin Date" field so I am not sure that adding this value to the message really adds value.