custom validator for C# web app

Question

In a C# 2010 webform application I am working on a couple of custom validators. One of the validators needs to look at a sql server 2008 r2 database to find the lowest and highest value for a create date field.
  My question is how to setup a customer error message that can be generated. Basically I would like the error message to say the create date needs to be within a certain date range. I would do not want to say the create date requested values is not contained in the specific table.
  Thus can you tell me how to setup a custom error message that the custom validator can use?