If shifting by work days rather than weeks, you will need to select which days of the week are 'work days' for you specifically (or which days you generally have work scheduled on. Only the selected days will be included in the shift. If you do not have Saturday selected as a work day, but have work scheduled on Saturdays, these jobs will not be moved in the shift. When choosing 'how many work days?', this will only include the selected work days.