at and crontab are command to schedule tasks on your Linux, this way repetitive tasks can be arranged so you can forget about them and be sure they are going to be executed, while cron jobs are used for repetitive tasks at is used for tasks that are going to executed in the future but only once.
Well, while there are ways to schedule this tasks from the command line, this time I will show you how to do it from the gnome-schedule, a front-end interface for both at and crontab.
Debian / Ubuntu
apt-get install gnome-schedule
Fedora / Centos
You need to have DAG repository installed and then:
yum install gnome-schedule
If you run this as root you will be able to schedule tasks for root.
On this first screen you can click on new to see this other dialog box
Here you select if you would like to issue an at (one time) or cronjob (recurrently) task.
If you select "A scheduled task that will launch recurrently" you will this other window.
Enter a Description, the recurrence, and the command that could a single command or a script you have previously written.
You can select the "Advanced" tab to have more control, about when the command is going to be executed, or if you are familiar with the sintax of cronjobs.
You will see this other window.
If you select the "Scheduled task that will launch only once in the future" you will see this window.
Here you should enter a Description and the script, which is actually the command you would like to execute in the future.
Then select the "Date and Time" tab
And adjust the date and time when you want the given task to be executed.
All the configuration you make with this tool, is stored at: