Execute a script on 4 week of the month

I need help to write a script and it will execute on Saturday of the last week of every month.
For example : 09/30/2006 is the last Saturday on Sept. 2006
08/26/2006 is the last Saturday on Aug. 2006
07/29/2006 is the last Saturday on July. 2006
etc..

Thanks,

weekselector extend the capabilities of a crontab