A lot of people are probably going to say - oh Christmas here is thus & so... (better, worse, different, etc)...but so many things are down to what a person's family does & people tend to characterise that as British, when it's really just individual family things - lol!
But I would probably bring up the Christmas pudding & Christmas cake, wearing the paper crowns, Christmas crackers & things like that. Oh & for music - the Christmas number one! Just trying to think of stuff that is genuinely British, as opposed to my family does this - therefore that's what British people do.
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Maybe turkey really being the traditional Christmas meal over here (I mean it was on the Life in the UK test!), whereas I think there is more variance in the US as to what you have for Christmas dinner.