You mean like last year when they clashed? EastEnders came out top on Overnights while Downton came just ahead of it in the officials. This will happen again this year.
Last year both EastEnders and Downton started at 9:00pm
Your making it sound like they have never clashed before and that this is the first xmas Downton has gone head to head with EastEnders.
EastEnders could be back to the levels of last year on Christmas Day but that's going to be quite a stretch. Not impossible but very difficult.