Not a crontab solution but have a look at iCal alarms. I know they can be set to run when iCal isn't itself open but I don't know if they run when that user is not logged in.

I need to run some things at 22.00 on every 3rd Saturday of every month, would the following work? 00 22 15-21 * * [ `date +\%u` = 6 ] && /root/test.sh i would use 6 instead of Sat so that if someone ...