Buenos Dias - kinda covers it all dont it?
As for the dating well, lets see - according to the account of the birth, it was when the Romans were collecting taxes that Joseph and Mary travelled to Bethleham, and the Romans collected taxes mid year.
Christmas got moved to include the Mithratists - another group who said that the savior was come to save mankind, but who held that the holy of holy days was 25 Dec - when he was born.
And as for the Early Church being adaptive, look at the whole structure, and replace God with Gods, and you\'ve got the structure of the Roman system - including the lack of marriage/celabacy by some of the hierarchs.