Well, it's only common in some regions of the world where the religion is more practised... (our region is a bit lax about that in later years)
as for the december january thing... If I remember correctly it depended on which division of religion you're in.
Depending on which religion or division of religion you follow they either follow the 'december' month as being christmas, but I know that due to historical calculation errors it's possible to also see it as january... So some follow that line... (If I remember correctly, some countries also decided it would be easier to celebrate in january since december already had other festivities)
I'm not quite sure, but that's what I remember anyways...
(and I've sent the trade. sorry it took me a while)