Why do we (Christians) celebrate the birth of Christ now althought some sources say that he was born in September or October?
Bonus question; Why do some atheists will be disappointed if they do not get Christmas presents?
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Since when did credible fact play any part in what Christians celebrate and when? Lets face it, Christians would celebrate the breaking of Mary's hymen if they came up with a holiday for it.
As for your second question, let me answer it in a different way. I have a nephew, his family are not religious but all his young life he has looked towards xmas as the time when Santa comes to visit, leave him presents and toys. It is how he has been raised simply because in the mainstream, that is what xmas is about. Where do we see Jesus being used as a marketing figurehead?
Religion and the Christian god will be dead in a few generations, Santa will reign supreme standing over the broken cross of his fallen rival. Save for a few sad followers, who as their lives slowly come to an end realise they have wasted their lives on a belief of something on par with the Easter bunny.