Q. On the Christmas eve, for one that keeps the Nittel tradition of not learning Torah on that night but usually has a shiur on the evening, what hours exactly are the ones one should avoid learning?
A. Nitei Gavriel (Chanuka p. 398) quotes Chidushe Harim that the traditional Nittel learning prohibition applies mainly from the beginning of the night until midnight (chatzos).
Chayei Yitzchok (ibid.) mentions that some begin Nittel from the chatzos of the day, the midday of the 24th.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a