The 5th working day could be any date from the fifth through the ninth, and maybe longer if there were a two week plant shutdown for instance.
Use the holiday file concept, but only include the dates that the job should run on, and run the cron job every day.
Create a directory workday, and include files in it named 20100924 etc. Maybe include as data in the file, the job steps to be run that day.
Then:
Code:
00 08 * * * [ -r /workday/`date +"%Y%m%d"` ] && job