Why do some Christians not understand that December 25th can be celebrated by Atheists?

Many Christians do not understand that Christ was not born in December, they do not understand that the holiday was celebrated then in order to stamp out the major 'competitive' religion of the time, paganism. Pagans celebrated Yule on the 21st, eventually late December became widely thought of for Christmas, not Yule, Winter Solstice or any other holiday that was once celebrated. Because many Christians (as I was) were never taught of the more bloody portions of their history they never knew. Therefore they see Christmas as 'theirs'. They have been taught that Jesus is 'the reason for the season', and find it difficult to see why anyone would celebrate when they do not believe in the 'reason'. Easy to see why they are confused, ironic though, that even they too are celebrating due to tradition as opposed to the actual event.