[CentOS] run script on cron job only run on first Saturdat every month???

we have CENTOS 5.5 on DELL server.  One of our script need run on first Saturday every month.

 We have following setup on cron job but it run every Saturday.  

15 04 1-7 * 6 /home/app/oracle/backup/monthlybk.sh

  Any one know how to fix it?

Thanks.