Backup system using cron

Since all backup plugins does not let me to choose specifc websites on my wpms to be backed up I did an sh script to be run through CRON, which is set in it as 0 0 * * 6 “script-path”, as said at http://stackoverflow.com/questions/16717930/how-to-run-crontab-job-every-week-on-sunday

However it does not seem to be working, maybe because of the “zeros”? Any idea? Thank you!