I think Christmas is likely to be a difficult day and emotional anyway, especially if you haven't spent a Christmas with him for a while. I really like the suggestion of telling him a day or two beforehand if you can manage it. Yes he might find it difficult, but he might also find it comforting in a bitter-sweet way that life continues and the family carries on, and the birth of a child brings such deep joy. I think it's better to tell him earlier as it will only be a few months before your pregnancy is visible and then it could be really awkward not to have said anything earlier. Good luck. It might not be easy but I'm pretty sure he will appreciate that you made the effort to tell him personally.