Função para descobri voce esta qual dia da semana

create FUNCTION  F_CONTADIA (dia NUMBER, data_ini DATE, data_fim DATE) RETURN
NUMBER IS
  datax DATE;
  cont NUMBER;
BEGIN
  datax:= data_ini;
  cont:= 0;
  WHILE datax <= data_fim LOOP
    IF TO_NUMBER(TO_CHAR(datax, 'D')) = dia THEN
       cont:= cont + 1;
    END IF;
    datax:= datax + 1;
  END LOOP;
  RETURN(cont);
END;

SELECT SUM(Sabado_domingo) Sabado_domingo
       FROM(select f_contadia(6,SYSDATE,sysdate)Sabado_domingo from dual
            UNION ALL
            select f_contadia(7,SYSDATE,sysdate)Sabado_domingo from dual);