SQL> SELECT themonth
  2       , count(theday) cnt
  3    FROM (  SELECT TO_CHAR(TRUNC(SYSDATE,'YY')+LEVEL-1,'Month') themonth
  4                 , TO_CHAR(TRUNC(SYSDATE,'YY')+LEVEL-1,'DY')    theday
  5              FROM dual
  6           CONNECT BY LEVEL <= ADD_MONTHS(TRUNC(SYSDATE,'YY'),12)-TRUNC(SYSDATE,'YY')
  7         )
  8   WHERE theday NOT IN ('SAT','SUN')
  9   GROUP BY themonth
 10   ORDER BY TO_DATE(themonth,'MM')
 11  /

THEMONTH         CNT
--------- ----------
January           22
February          20
March             23
April             20
May               23
June              22
July              21
August            23
September         21
October           22
November          22
December          21

12 rows selected.

SQL> SELECT SUM( -- transform the generated date to abbreviated day of week
  2              DECODE( TO_CHAR(TRUNC(thedate,'MM')+LEVEL-1,'DY')
  3                   , 'SAT', 0 -- Saturdays don't count
  4                   , 'SUN', 0 -- Sundays   don't count
  5                   , 1        -- all other days count
  6                   )
  7            ) sales_day_no
  8       , thedate
  9    FROM ( SELECT TO_DATE('22/02/2006','DD/MM/YYYY') thedate
 10             FROM dual
 11         )
 12   -- row generator technique from 1/2/2006 to 22/2/2006
 13   CONNECT BY TRUNC(thedate,'MM')+LEVEL-1 <= thedate
 14  /

SALES_DAY_NO THEDATE
------------ ---------
          16 22-FEB-06

CREATE OR REPLACE FUNCTION get_sales_day_no(pid_date IN DATE)
  RETURN NUMBER DETERMINISTIC
IS
  v_return NUMBER;
BEGIN
  SELECT SUM( -- transform the generated date to abbreviated day of week
              DECODE( TO_CHAR(TRUNC(pid_date,'MM')+LEVEL-1,'DY')
                   , 'SAT', 0 -- Saturdays don't count
                   , 'SUN', 0 -- Sundays   don't count
                   , 1        -- all other days count
                   )
            ) sales_day_no
    INTO v_return
    FROM dual
 -- row generator technique
 CONNECT BY TRUNC(pid_date,'MM')+LEVEL-1 <= pid_date;
 
 RETURN v_return;
END get_sales_day_no;
/
sho err

SELECT TO_DATE('22/02/2006','dd/mm/yyyy') yourdate
     , get_sales_day_no(TO_DATE('22/02/2006','dd/mm/yyyy')) sales_day_no
  FROM dual
/

DROP FUNCTION get_sales_day_no
/

SQL> @C:\useful\orafaq

Function created.

No errors.

YOURDATE  SALES_DAY_NO
--------- ------------
22-FEB-06           16


Function dropped.