By Michael Keller on Wed, 1999-09-01 01:00. HOWTOs
I found this site helpful
http://www.linuxjournal.com/article/3290
and this on
http://www.pantz.org/software/cron/croninfo.html
Cron is a daemon that executes scheduled commands. Cron is started automatically from /etc/init.d on entering multi-user runlevels. Cron searches its spool area (/var/spool/cron/crontabs) for crontab files (which are named after accounts in /etc/passwd); crontabs found are loaded into memory. Note that crontabs in this directory should not be accessed directly - the crontab command should be used to access and update them.
Crontab is the program used to install, deinstall or list the tables used to drive the cron(8) daemon in Vixie Cron. Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly.
This line is and example of running a cron job every month, on Mondays whose dates are between 15-21. This means the third Monday only of the month at 4 a.m.
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0 4 15-21 * 1 /command
This line executes the "ping" and the "ls" command every 12am and 12pm on the 1st day of every 2nd month. It also puts the output of the commands into the log file /var/log/cronrun.
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0 0,12 1 */2 * /sbin/ping -c 192.168.0.1; ls -la >>/var/log/cronrun