This Halacha is an excerpt from our Sefer
Some are accustomed to fast during the [daylight hours of the] three weeks, from the 17th of Tammuz.
Mourning after midday:
Some have the custom to mourn and cry over the destruction of the Temple for a half hour or more each day, after midday, throughout the days of the 17th of Tammuz and 9th of Av. Doing so is a proper custom, as the three weeks is a time of severities, and this mourning benefits the soul tremendously. Some say Tikkun Rachel at this time. This is addition to performing Tikkun Chatzos at night.
Learning the laws of Beis Habechira/The Temple:
The Midrash states that studying the laws of the Beis Hamikdash, its structure and vessels, is viewed by Hashem as if one is building the Temple. This means to say, that during times of exile when one is unable to help build a physical Temple, one fulfills the eternal Biblical command of “Making me a Temple” through studying the laws of the Temple. One should study the prophecies in Yechezkal from 40 and onwards, Miseches Middos, and Hilchos Beis Habechira of the Rambam. One should especially increase in studying these laws during the period of Bein Hametzarim. Studying these laws weakens the exile and hastens the redemption.
 Michaber 551:16; Tur in name of Avi Ezri; See Kaf Hachaim 551:191-192
 Arizal in Shaar Hakavanos p. 89, brought in M”A 551:45; Peri Eitz Chaim 23:1; Elya Raba 549:4; P”M 549 M”Z 1; Ateres Zekeinim 550; Kaf Hachaim 551:221-224
 Shaar Hakavanos ibid; Ateres Zekeinim ibid
 Yosef Ometz 21; Kaf Hachaim 551:222
 See Likkutei Sichos 18:412; Hisvadyus 5748 2:564; Shulchan Menachem 3:52
 Tanchuma Tzav 14; Pesikta of Rav Kahana 6; Pesikta Rabasi 16; Yalkut Shimoni Yechezkal 43:10
 Toras Menachem 5750 4:55