To use the crontab command, users must be listed in the file /etc/cron.allow. If that file doesn't exist, the users must not be listed in /etc/cron.deny. If neither file exists, all users can use the crontab command unless some other configuration prevents this.

e.g. 00 20 * * 1-5 /usr/sales
Time pattern in a crontab file entry include minute, hour, day of month, month of year, and day of week.

$ crontab -l
To list the available crontab enteries.

$ crontab -e
To edit ( or add ) the crontab enteries

$ crontab -e to create
To create the new crontab, if it doesn't exists.