Today (Dec 24, ~11AM local time, 10AM UTC) when I voted on a post (both Ask Ubuntu and Meta SE), I received the "Bûche de Noël" hat on both sites.
However, the description of that hat on http://winterbash2016.stackexchange.com/ reads
post or vote on December 25
The hat I expected to get instead is "Have a little Dreidel", which should be awarded for
post or vote on any of the days of Chanukah
On the Meta Answer here, this is more detailedly specified as Dec 24 - Jan 1.
Why am I getting the wrong hat here? Bug in the hat system? Wrong hat descriptions?
Update: I got both hats awarded now on all qualifying sites, so the bug seems to be fixed and the provided explanations seem correct as provided in the accepted answer.