What attribute must be set on a validator control for the validation to work ?

Use the ControlToValidate property to specify the date control to validate. This property must be set to the ID of a date control for all validation controls except for the CustomValidator control, which can be left blank. If you do not specify a valid date control, an exception will be thrown when the page is rendered. The ID must refer to a control within the same container as the validation control. It must be in the same page or user control, or it must be in the same template of a templated control.