The three weeks beginning with the 17th of Tammuz and ending with the Ninth of Av are known as Bein haMetzarim ("between the straits", i.e. between the days of distress), or The Three Weeks. Some customs of mourning, which commemorate the destruction of Jerusalem, are observed from the start of the Three Weeks.
The oldest extant reference to these days as Bein haMetzarim – which is also the first source for a special status of The Three Weeks – is found in Eikhah Rabbati 1.29 (Lamentations Rabbah, fourth century CE?). This midrash glosses Lamentations 1:3, "All pursuers overtook her between the straits."
The three weeks of mourning between the 17th of Tammuz and 9th of Av is cited as a rabbinically instituted period of fasting for the "especially pious". Such fasting is observed from morning to evening, common with other rabbi-decreed fasts.
Read more about this topic: Seventeenth Of Tammuz