The twenty-one day period between the seventeenth of Tammuz and the ninth of Av, two days of misfortune and distress, is referred to as bein hametzarim—"between the straits."
Shining a Beacon of Mitzvah Light on Days of Darkness
The Lubavitcher Rebbe spoke of the need to shine a beacon of light on these days of darkness through participation in a siyum during each of these nine days.
The haftorahs of the Three Weeks tell of G‑d’s disappointment with Israel’s faithlessness, and the impending doom that would befall them if they would not mend their ways.