Q. Why do the ones that keep Nittel do so on the Goishe date and not in the Jewish one as we do with everything else?
A. Siach Sarfei Kodesh (p. 192) relates that when the last Lubavitcher Rebbe zt’l was sitting shiva, the Satmar Rebbe zt;l, Ha-Rav Yoel Teitelbaum, paid a shiva call and asked him: Why do we observe the night of “Nittel Nacht” according to the non-Jewish calendar (which is forbidden) instead of according to the Jewish calendar? The Lubavitcher Rebbe answered that we do not learn on that night in order not to give spiritual strength to the god of the Christians and we therefore observe it on the night that they celebrate, which is according to their calendar. And – he added – we must therefore be aware when they celebrate this holiday in the place we are located. In most places, their holiday is on December 25th, but it other places it is on a different day (For example, January 6th). In such places, that is the night of “Nittel Nacht”. (From Toras Harav Aviner).
Rabbi A. Bartfeld