I think you can concentrate on the midwinter festival side of it, family and friends, celebration, presents, etc.
And I also don't see a reason not to celebrate it as the birth of Jesus, even if much about the actual story isn't true - I tend to think of it as being based on some events that probably happened, to a very rough extent, and that he was an influential person and did some good things, regardless of what people did in his name afterwards, regardless of whether every aspect was good, regardless of whether someone believes the Son of God part, etc. So I can make peace with the idea that people might want to do something to celebrate his birth, without having to believe in or agree with much of the Christian view of things.
And I can see the story as being an allegory rather than literal truth, so don't mind singing carols about highly improbable events.
I can also appreciate the historical aspect of it, that it's something that's been celebrated in the culture for a long time, and has changed in all sorts of ways over time, but it can still be seen as part of my culture - the lack of religious belief is part of those changes. Many cultures have some unusual traditions that probably at one point involved belief in something that most people there no longer feel is true, but they still enjoy the traditions, food, costumes, etc as a part of their heritage that they can be proud of.
And traditions like Santa/Father Christmas can be brought into that. Tradition of charity, peace, love, etc are valuable whether there is a God involved or not.
anyway, that's how I reconcile it all