The Rebbe's guidance on the Three Weeks (Bein HaMetzarim) and Tisha B'Av:
Transforming the days of mourning. Since the destruction came through baseless hatred, the Three Weeks call above all for increased Ahavas Yisrael — love for every Jew, even those we may disagree with — which repairs the cause of the exile (IK 2189, 7709, 8869). Even in days associated with mourning, keep up and increase one's good activities (IK 5617, 5626). Reflecting on the destruction must lead to practical improvement in Torah and mitzvos (IK 8870).
What to add. Intensify Torah study during the Three Weeks (IK 7719), spread Chassidus, which is especially powerful now (IK 6950), and support Torah education, which addresses the root cause of exile (IK 7705). Farbrengens during this time have their place (IK 9524). Even young children have a special role, through Torah learning and tzedakah, in bringing the redemption (IK 10830, 10848).
Comfort. True comfort comes from G-d Himself, and it is near as Menachem Av approaches (IK 8476). Tisha B'Av and Yom Kippur share a deep connection, and Tisha B'Av is itself called a "festival" (IK 1118, 7030).
Fasting. The fast of Tisha B'Av is obligatory; minimize exertion and seek practical solutions where health requires (IK 6951). On netilas yadayim on fast days, see IK 506.
See also: Shabbos and holidays – Chassidic holidays; Emotional well-being – Anger, resentment and hurt feelings.