Personally, I've always considered a white Christmas to only happen when there is any real amount of snow lying on the ground on 25th Dec, no matter whether or not any snow actually falls on that day.
IIRC we didn't have any snow falling here on Xmas Day in either 2009 or 2010, but there was still a great deal of snow on the ground on both occasions so both were white Christmases as far as I was concerned.
Originally Posted by: David M Porter