WORKDAY(start_date, days, holidays)

CREATE OR REPLACE FUNCTION workdays(start_date DATE, days NUMBER)
  RETURN DATE
IS
  i   NUMBER := 0;
  tot NUMBER := 0;
BEGIN
  IF days = 0 THEN
     RETURN start_date;
  END IF;

  WHILE true LOOP
    IF days > 0 THEN
       i := i + 1;
    ELSE
       i := i - 1;
    END IF;

    IF TO_CHAR(start_date + i, 'FMDAY') NOT IN ('SATURDAY', 'SUNDAY') THEN
       tot := tot + 1;
    END IF;

    IF abs(tot) = abs(days) THEN
       exit;
    END IF;
  END LOOP;

  RETURN start_date + i;
END workdays;
/

with src as (select sysdate dte
                   ,level   days 
             from dual
             connect by level <= 15)
select dte + days + (floor(days/5)*2) 
       + CASE to_number(to_char(dte + days + (floor(days/5)*2),'D','nls_date_language=english'))
              WHEN 6 THEN 2
              WHEN 7 THEN 1
              ELSE 0
              END work_day
       ,dte
       ,days
from src;