|The 3 weeks|
Nshei Chabad Newsletter / The Rebbe repeatedly exhorted us to learn about the Bais Hamikdosh during the Three Weeks* because it is written that learning about the Bais Hamikdosh is considered like building it.
He also spoke about increasing our learning of Torah in general and giving Tzedokoh during the Three Weeks, as it says, "Tziyon B'mishpot (referring to Torah) Tipodeh V'shAveho B'tzedokoh." Giving Tzedokoh is clearly one of the ways to hasten the Geulah and the rebuilding of the Bais Hamikdosh.
During the Three Weeks we don't say Shehecheyonu, not even on Shabbos (Sefer Haminhogim, page 46).
*The Three Weeks are the period of mourning for the Bais Hamikdosh from the 17th of Tammuz through the first nine days of Av, culminating in Tisha B'Av. The Nine Days are included in the Three Weeks: From the first day of Av (Rosh Chodesh) through Tisha B'Av. Historically, these are days of hardship and catastrophe for the Jewish people. Both the First and Second Batei Mikdoshos were destroyed during this time, among other tragedies, so the entire Jewish people is in mourning. We do not make weddings, listen to music, take pleasure trips, or hold public celebrations except for a Seudas Mitzvah such as a Bris Milah or a Siyum.