I believe I understand better. You are wanting a query to run after the parameter has been selected. That is not quite how the parameters work as they query before in order to populate the list boxes. However, you can actually handle your situation in one of 2 ways. Create a second dependent parameter (which can be hidden) which takes the value of the selected date type parameter and runs your query to get the dates or you could add the date query to your main query and set the variables for your dates there based on the selected date type. I would prefer the first method.