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.
There are two sorts of inner convictions of the soul, this thing we call faith.
Unbound by the confines of space and time, your soul sees a reality your mind cannot fathom. From that vision seeps down a conviction that G-d is with you, that He is good, and that there is really nothing else but Him.
Not because you understand. But because your soul sees that this is so. And she sees with a clarity and certainty the flesh eye could never attain.
Nevertheless, a higher vision means there are two and not one: There is you and there is the vision you perceive. And if there are two, two can be separated.
So that, when darkness and confusion swells and storms, threatening to rip you away from your G-d, a higher vision is not enough.
That is when you need to reach to the very core of your soul. Not to that place in the soul that sees G-d, but to the essence of the soul that is truly a part of G-d.
To say, "This is my G‑d. I am His, He is mine, and we are one."
"And so, nothing can stand between us."