Of course it isn't arbitrary - it's a combination of Saturnalia and any number of sun-rebirth traditions. It just isn't the birthdate of Jesus - or at least there's no reason to believe it was. If you want to get into debates about the conception date of John the Baptist and what times his father was required at temple taht's another issue entirely...
As for the year - Dennis the Short's dates cannot be correct if the Gospel's are correct and Herod was alive at the time of Christ's birth. Since the latest date that Herod was alive was 1 BC and the most likely year of death is 4BC then the year's cannot be right can they?
Pete
God is our guide! from field, from wave, From plough, from anvil, and from loom; We come, our country's rights to save, And speak a tyrant faction's doom: We raise the watch-word liberty; We will, we will,we will be free!