(a) Asarah BeTeves:

Regarding the fast of the Tenth of Teves, see the above section (p. 92ff.) on The Three Weeks and Public Fasts.

(b) Restriction on Torah Study:

The Rebbe Rashab once said that the reason for not studying Torah on Nittel-nacht, [the Yiddish name for the night before 25th December,] is that “one should not elicit an increase in life-force.” This restriction on study applies only until midnight.1

(c) Shabbos Shirah:

On Shabbos Shirah [i.e., the Shabbos on which one reads Parshas Beshalach, which includes the Shirah, the Song of the Sea; Shmos 15:1-19], it is customary to eat black buckwheat (kashe).2

(d) Chaf-Daled Teves:

On the 24th of Teves the Alter Rebbe passed away in the village of Piena. [The Alter Rebbe, R. Shneur Zalman (Boruchovitch) of Liadi, born on Chai Elul 5505 (תק"ה; 1745), was the founder of the Chabad-Lubavitch stream within the chassidic movement, which he led until his passing in 5573 (תקע"ג; 1812). His resting place is in Haditch.]

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“And now, because of our many transgressions, the sun has become dimmed at midday; ‘yearning for its place, it has set,’3 having been ‘summoned to the Academy on High.’4 ‘The angels have been victorious; the holy ark has been hidden away.’5 ‘His soul departed in purity,’6 in the wake of awesome wonders which we witnessed during those days that preceded his passing. Until the day of his rest ‘his mouth did not cease from the study of Torah.’7 With a clear and tranquil mind, and cleaving wondrously to his Maker, he prayed Maariv; he recited Havdalah in the blessing of חונן הדעת, [blessing Him Who grants understanding]. Then, on Motzaei Shabbos of Parshas Shmos, on the 24th of Teves 5573, ‘he was united in a perfect bond’8 with the Holy One, blessed be He.”9

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“[On Rosh Chodesh] Elul he travelled with his family to Krasna. On Friday, the following erev Shabbos, they fled from the French [i.e., Napoleon’s invading forces] and continued wandering [for over three months] until they arrived in Piena on erev Shabbos, Parshas Vayishlach. There, at the conclusion of Shabbos Parshas Shmos (23rd Teves), in the middle of the eleventh hour on the eve of the 24th, the holy ark — the luminary of Israel, G‑d’s anointed one — was captured, and was ‘summoned to the Academy on High.’660

“May G‑d hasten the coming of our Redeemer, when ‘those who repose in the dust will awaken and sing joyful praises.’10 And he (the Alter Rebbe) amongst them will relate to us the wonders of ‘G‑d’s perfect Torah, which restores the soul’11 in the body — for ‘the dew of the Torah revives.’12 May this indeed be G‑d’s will, speedily and soon!”13