No, the voucher does not need to be spent the day you receive it. Please just ensure that you've chosen a supermarket brand before the Huggg link expires.
Once you've chosen, you'll be directed to the supermarket voucher which will have an expiry date set by the brand. This is the date by which you must use your voucher.
Please note that you can use the voucher towards multiple visits, the balance does not have to be used all at once.