Here is a simple example. First create a stored procedure in your databaseCREATE PROCEDURE dbo.GetDateName
@date DATETIME
AS
SELECT DATENAME(weekday,@date);
GO
Now use that in a report as the data set. You will have a parameter @date, for which you can select the date and the report will show the weekday name.
Now replace the contents of the stored proc with what your actual query is.