Shmarya Rosenberg reports:

Tonight is Christmas Eve – better known in the haredi world as Nittel Nacht. Hasidic Jews believe that evil spiritual forces are at full force tonight and to prevent giving strength to any of these impure forces, hasidim do not study Torah, Talmud, halakha or other Jewish religious subjects, and they also abstain from having sex.

The 6th Lubavitcher Rebbe, left, playing chess with his younger son-in-law, the future 7th Rebbe, right, allegedly on Nittel Nacht

Nittel Nacht: No Sex, No Torah, Just Fear Of Evil Spiritual Forces Associated With Jesus

Tonight is Christmas Eve – better known in the haredi world as Nittel Nacht.

Primarily hasidic Jews believe that evil spiritual forces are at full force tonight and to prevent giving strength to any of these impure forces, hasidim do not study Torah, Talmud, halakha or other Jewish religious subjects, and they also abstain from having sex.

Ha’aretz reports that Sefer Haminhagim, The Book of Religious Customs, teaches that most heretics who converted from Judaism to other religions, especially Christianity, were conceived through “illegal” sex by their parents on Christmas, the first Christian’s birthday.

So what do hasidim do on Christmas? They don’t – officially, at least – have t.v.s or go to movies. The Internet is banned. So is reading for pleasure. And for tonight, so is Torah study of all types and sex. So what is it that hasidm do?

Several things:

Play chess or card games.

Rip toilet paper for Shabbat. Since Orthodox Jews don’t rip toilet paper on Shabbat, it has to be ripped beforehand (or tissues must be uses in its place). The lore is that ome hasidic rebbes used to spend all of Christmas eve ripping toilet paper for the next year (although it may be more likely the Rebbes’ butlers, footmen or other attendants actually did the ripping.)

Pay bills, go over charity accounts.

Do mundane tasks that need to be done around the house.

One of the reasons the custom of not learning Torah tonight may have developed was a need to help prevent anti-Jewish violence. Prohibiting Torah learning meant the synagogues were closed and dark, and Jews were, for the most, inside their homes and not out on the streets, making anti-Jewish violence that often broke out on Christian religious holidays somewhat less likely.

The name Nittel Nacht could have its origins in the Latin name Christmas had in the Middle Ages – “Natal Domini,” “the birth of the Lord.”

The Sefardi haredi Shas Party’s journal Yom Leyom published an article claiming that this is really a confusion of the Latin name for Sylvester night (New Year’s Eve) Natlus.

The hasidic newspaper Hamodia claims that, “because they did not want to say the name of that same man [Jesus], the name of the wicked shall rot, they called him the hung man [“natal” means “hanged” (or less plausibly, “to have been taken away”)], as he was killed and hung from a tree,” following a later – and almost certainly incorrect – Talmudic tradition that Jesus was hung, not crucified.

There are also claims that the name is an acronym.

And, of course, this wouldn’t be haredi Judaism if there wasn’t an argument over the observance. Some groups observe Nittel Nacht tonight, following the observance of Christmas by the Roman Church. Others will observe it next week, following the observance of Christmas then by the Eastern Churches. Others will observe it on both nights just be absolutely sure. 
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More info here: https://theuglytruth.wordpress.com/2012/12/31/how-hasidic-jews-celebrate-christmas/

and here: http://theinfounderground.com/forum/viewtopic.php?f=40&t=17009

also:

Photos of Nittel Nacht in Crown Hights: http://www.crownheights.info/index.php?itemid=49259