Because of the holiness of Shabbat, the fast of Tammuz 17 is this year postponed to today, Tammuz 18. We refrain from all food and drink from "daybreak" (about an hour before sunrise, depending on location) until nightfall. Special prayers and Torah readings are added to the day's services.
The fast day mourns the breaching of Jerusalem's walls and the other tragic events that occurred on Tammuz 17--see "Today in Jewish History" for that date--and repenting and rectifying their causes.
Link: Halachic times for today's fast
During the Three Weeks, from 17th of Tamuz to the 9th of Av, we commemorate the conquest of Jerusalem, the destruction of the Holy Temple and the dispersion of the Jewish people.
Weddings and other joyful events are not held during this period; like mourners, we do not cut our hair, and various pleasurable activities are limited or proscribed. (The particular mourning customs vary from community to community, so consult a competent halachic authority for details.)
Citing the verse (Isaiah 1:27) "Zion shall be redeemed with mishpat [Torah] and its returnees with tzedakah," the Rebbe urged that we increase in Torah study (particularly the study of the laws of the Holy Temple) and charity during this period.
Links:
The Three Weeks
Look deeply and you will see that the Torah does not know of man and woman as separate beings.
Each act is performed once through a single body—a body that in our world may appear as two, but which the Torah sees as one.
On the contrary, for both to be assigned the same mitzvah would be redundant, for why should one half of the body do that which the other has already accomplished?
Just as a man fulfills the mitzvah to be fruitful and multiply through the agency of his female counterpart, so does a woman wrap tefillin or wear tzitzit on the body of her male counterpart.
For just as man and woman were first created as a single form, so too, before each soul descends below, they begin as one.
It may be at times that only half a soul must descend for its divine mission, while the other half waits patiently above. And when it will return, they will merge once again.