Before you can schedule a job in Job Definitions you need to have at least one draft of a job created and the data can't have been loaded from the local computer. For more information, see topic Load data with Data Loader.
If you want the data loading job to run on a specific day you can do it during the creation of the job. But if you need to schedule a job that was already created you can do it in the Job Definitions page.
To schedule a job:
In the Job Definitions page select Drafts to see the drafts you created.
Select the icon on the right of the date for the job you want to edit.
Select Schedule.
Specify the required schedule.
In the following example, the job is scheduled to run on every Sunday: