Instructions:23rd November is this year's cut off date for the Asda Christmas Savings Card. Full details here http://christmassavingscard.asda.com/faqs but if you pre pay £144, Asda will give you an extra £6 free.
They put it on a plastic credit card sized card which you can swipe at the checkout.
If you were spending that at Asda anyway, then why not pick up £6 free ? You can apparently do this an unlimited number of times, with £144+6 on each card.