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.
This is my G‑d, and I will praise Him, the G‑d of my father, and I will exalt him.
Exodus 15:2
When He is only the G‑d of your father, He remains exalted and beyond your world. When you make Him your own G‑d, then you can truly praise Him with your heart.
Rabbi Isaiah Horowitz (16th century)
Only a fool will toss out the inheritance of many generations. But one who does not take ownership remains a child.
So it is with a material estate, so it is with the faith of our fathers and mothers. We must make it our own faith, as well.
And how do you make that faith your own? Ironically, through the power of your own mind.
Engage your mind to live by your faith.