contar sábados antes de 15 datas como dias úteis e pular outros

CREATE FUNCTION FN_CNT_Working_days(StartDate DATE,
                                    EndDate   DATE) 
RETURNS INT
BEGIN
     DECLARE WORKING_DAYS INT;
     SELECT
   (DATEDIFF(EndDate, DATE(CONCAT(YEAR(EndDate), '-', MONTH(EndDate), '-', 16))) + 1)
  -(FLOOR(DATEDIFF(EndDate, DATE(CONCAT(YEAR(EndDate), '-', MONTH(EndDate), '-', 16)))/7) * 2)
  -(CASE WHEN DAYNAME(DATE(CONCAT(YEAR(EndDate), '-', MONTH(EndDate), '-', 16))) = 'Sunday' THEN 1 ELSE 0 END)
  -(CASE WHEN DAYNAME(EndDate) = 'Saturday' THEN 1 ELSE 0 END) 
   +
   (DATEDIFF(DATE(CONCAT(YEAR(EndDate), '-', MONTH(EndDate), '-', 15)), StartDate) + 1)
  -(FLOOR(DATEDIFF(DATE(CONCAT(YEAR(EndDate), '-', MONTH(EndDate), '-', 15)), StartDate)/7))
  -(CASE WHEN DAYNAME(StartDate) = 'Sunday' THEN 1 ELSE 0 END)
     INTO WORKING_DAYS;

     RETURN (WORKING_DAYS);
END;