How to set Crontab Entry for different-2 scenario

# How to set cron
00 0,4,8,12,16,20 * * * sh /u001/scripts/GPASP_Job_email.sh > /u001/log/GPASP_Job_email.log 2>&1

#How to set a cron to run only and only once in his whole life for specific time.

At command in Unix