That should do it:
5 8 * * 6 <user> <command>
or for readability
5 8 * * Sat <user> <command>
documentation (man 5 crontab):
field allowed values
day of month 1-31
month 1-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)
Sat 8:05AM run find
# Minute Hour Day of Month Month Day of Week Command
# (0-59) (0-23) (1-31) (1-12 or Jan-Dec) (0-6 or Sun-Sat)
5 8 * * Sat /usr/bin/fi
Set up cron in Magento
In the Magento admin, go to: System > Configuration > Advanced > System > Cron (Scheduled Tasks) and configure cron jobs you wish to run.
You should know that Magento runs cron jobs even if you don't have a daily cron job configured. Whenever Magento receives a request, it checks if there are any cron jobs to be run. Therefore, having the daily cron job would only make sense if you had no requests for an entire day.
I hope this will help you.