Tammuz 12 is the birthday the sixth Lubavitcher Rebbe, Rabbi Yosef Yitzchak Schneersohn of Lubavitch (1880-1950). This is also the day on which he was liberated from exile to the Soviet gulag 47 years later (see below).
Links:
A short biography
More on Rabbi Yosef Yitzchak of Lubavitch
On the 12th of Tammuz of 1927, the sixth Lubavitcher rebbe, Rabbi Yosef Yitzchak Schneersohn, was officially granted release from his sentence of exile to Kastroma in the interior of Russia.
Twenty-seven days earlier, the Rebbe had been arrested by agents of the GPU and the Yevsektzia ("Jewish Section" of the Communist Party) for his activities to preserve Judaism throughout the Soviet empire and sentenced to death, G-d forbid. International pressure forced the Soviets to commute the sentence to exile and, subsequently, to release him completely. The actual release took place on Tammuz 13, and Tammuz 12-13 is celebrated as a "festival of liberation" by the Chabad-Lubavitch community.
Tammuz 12 is also Rabbi Yosef Yitzchak's birthday (see above)
R. Jacob ben Asher, son of R. Asher ben Yechiel (the Rosh), was one of the most prominent Torah scholars in medieval Europe. His classic work on Jewish law, Arba’ah Turim (known also as Tur), covers every area of Jewish life (in the post-Temple era), presenting the various opinions of previous authorities along with the author’s own decisions. A host of commentaries were written on this work, including one by R. Yosef Caro and another by R. Moshe Isserlis. These two commentaries formed the basis for the Shulchan Aruch (Code of Jewish Law), the definitive guide to Jewish observance until today.
R. Jacob also authored a popular commentary on the Torah, uncovering layers of hidden meaning in the text by way of gematria (and other close analysis of the texts).
Link: Rabbi Jacob ben Asher
In Chabad practice, the mournful paragraphs of Av Harachamim and Tzidkatecha Tzedek are omitted from the morning and afternoon prayers respectively.
During the summer months, from the Shabbat after Passover until the Shabbat before Rosh Hashahah, we study a weekly chapter of the Talmud's Ethics of the Fathers ("Avot") each Shabbat afternoon; this week we study Chapter Five.
When the Mighty Waters cover your head, suffocating the soul and the flame that burnt inside . . .
When raging torrents of confusion drag you away in their current, ripping you from your hold on Life . . .
Look deeper. Beyond the soul.
For the soul itself, as well as the flame it holds, are rooted in a serene G‑dly World of Emanation, a world of quietness and sublime harmony.
But the turbulence of this world is rooted even beyond that, stemming from a World of Confusion, of light unbounded and untamed, before the orderly emanation of defined being, “and the world was confused and void, with darkness over the face of the deep.”
So you must dig deeper than those roots, to find the coals from which the flame arose and the flint rock from which the spark was struck. Deeper, until you reach the primordial essence of the soul, beyond Emanation, beyond Light—even beyond the unbounded light of pre-creation. Where there is nothing but the seminal thought which inspired all that is and was and will come to be.
And what was that thought? It was the thought of you here and now, in your struggle with this world, and the delicious taste of your victory.
And, since in that thought there is no past and future, there, in that thought, you have already won.
Now you must make it happen.