This article explains how to schedule a voucher to send at a later date.
Select the students you wish to assign the vouchers to. You have the option to filter the students by class or year group should you need to.
Click 'Assign vouchers' and then 'Send later'.
Select the scheduled send date and click 'Schedule vouchers'.
The status of these vouchers will now display 'Scheduled' and will show the date you selected.
If you select the incorrect schedule date, you will need to unassign all vouchers with that send date and then assign them with the correct schedule date.