Hope I'm not teaching granny, but unless your bed is covered/protected from frost I wouldn't try and grow Christmas spuds there. They are usually grown in bags that you can move inside if frost is on its way.
If you buy from a supermarket it may also be best to use bags, because eating (as opposed to seed) potatoes aren't certified virus free there's an outside chance of infecting your soil.
I grew Vivaldi and Anya from Sainsbury's in bags (September 08). The frosts were so heavy that despite being fleeced, they died back and didn't produce until about May this year (and not a very good crop!) but if looked after they, or something like Nicola or Charlotte should be ok.