The 17th of Tammuz (this Sunday, July 13, 2025) also marks the beginning of The Three Weeks period of mourning which culminates on the 9th of Av, commemorating 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. (Consult the Code of Jewish Law (Shulchan Aruch) or a qualified rabbi regarding specific proscriptions).
The Lubavitcher Rebbe urged that the Three Weeks should be a time of increased giving of charity and Torah study (in keeping with the verse (Isaiah 1:27), "Zion shall be redeemed by law, and her returnees by charity"), particularly the study of those portions of Torah that deal with the laws and the deeper significance of the Holy Temple.
Links:
TheThreeWeeks.com
Some Laws and Customs of the Three Weeks
About Holy Temple