From mcvuk.com:

Any machines that need fixing after tomorrow will not be returned until after the festive break.

European Xbox Community Manager Graeme Boyd has today confirmed via Twitter that any machines sent off to Germany for repair after December 11th will not be returned in time for Christmas.
"From the Xbox Support Team - if you're getting your Xbox repaired tomorrow is the last day for pick-up to get it back before Christmas," he explained.
Full Story: mcvuk.com