Nittelnacht is an anomoly in Jewish calendar as it commemerates an event in Christianity. It occurs in the evening of the 24th of December every year - Christmas Eve.
Nittelnacht is a word of interesting history. 'Nittel', the yiddish word for Christmas is almost certainly a derivative of the latin 'Natus', birth. The same word became Noel. However, the Yiddish word also carries another meaning. 'Nit' means 'Nothing' in Yiddish, so with the diminutive -l ending, Nittlenachet becomes "The night of little nothing".
Jews in the shtetls Eastern Europe had a number of customs on Nittelnacht. This was the only night where the yeshivot, Houses of study, synagogues and all communal buildings closed early. Jews wouldn't go out, and wouldn't even study at home.
Why? Because of the likelihood of drunken Christians descending for a pogrom, attacking those who they believed responsible for killing Jesus. Such attacks often happened, and the communities responded by keeping quiet and being ready.
So what did they do then? Well, with no studying, they had to make their own entertainment. They often played cards. Card playing was considered as a form of gambling (or at least as a temptation to gamble) so devout Jews would never do it on the rest of the year. But on Nittelnacht, it was allowed. Quite why this should be the case isn't clear - one source says it was to keep the Jews alert in case of an attack, but this seems a bit unlikely.
I've also heard that some Jews fasted on that night, as a sign of sorrow for the start of major troubles for the Jewish people, though I can't find any sources to that effect.
Nowadays, Nittelnacht certainly isn't observed outside the Haredi communities, and probably even by them either. Some of its customs - like playing cards - have been incorporated, more positively, into Hanukah.
http://www.sdv.fr/judaisme/traditio/hanouka/hanouka.htm
http://www.forward.com/issues/2002/02.12.27/arts4.html
Suitable candidate for The Ninjagirls' quest