An awesome prayer for the fast of the 17th of Tammuz, based on the secret of the day's greatness, the events that occurred on it, and what can be achieved on this great and awesome day:

PRAYER FOR THE DAY OF THE 17TH OF TAMMUZ

Master of the Universe, Almighty One, on the 17th of Tammuz—the day on which the Malchus (Kingship) of Atzilus (the World of Emanation) is revealed through the rectification of the five things: A. The first thing, as it is said, "And it came to pass, as he drew near to the camp and saw the calf, he cast the tablets from his hands." And it is written that in the fourth month, on the ninth of the month, the city [walls] were breached, and then the King [Zedekiah] went out from between the two walls and went by way of the Arabah, and they overtook him in the plains of Jericho. They spoke judgment with him, slaughtered his sons before his eyes, and blinded the eyes of Zedekiah. Then He "called a festival against me" (a reference to the destruction), and Yehoyachin ascended to the roof of the Heichal (Sanctuary) and threw the keys upward, and a palm of a hand emerged and received them. B. And the tribe of Benjamin was nearly annihilated, with only 600 men remaining, among them Saul. C. And the Tamid (daily offering) was abolished. D. And Turnusrufus burned the Torah. E. And Menashe placed an idol in the Heichal (Sanctuary), for Menashe would slaughter 1,000 people a day for 22 years. And now, on the 17th of Tammuz, when the Malchus (Kingship) of Hashem—the Malchus of Atzilus—is revealed eye to eye through the opening of 48 million gates of Gehinnom (Purgatory). Through them, the wicked who caused the destruction of the Holy Temple are brought out, twenty-one days before Tisha B'Av. Hashem nullifies from those wicked ones the judgment of "their worm shall not die and their fire shall not be extinguished" twenty-one days before Tisha B'Av, and immediately they see the face of the Shechinah (Divine Presence). And the Tzaddikim who never sinned, yet were presented with endless sins they did not commit, receive their reward on that day twofold, in the secret of "for she has received... double for all her sins." Whereas the Ba'alei Teshuvah (penitents) who broke their hearts before Hashem for every single sin will merit to resurrect the dead, as it is said, "And when the wicked man turns away from his wickedness... and does what is lawful and right, he shall live by them." It does not say "in them" but "by them" (or "upon them"), teaching that through them he shall live eternal life, like Serach daughter of Asher and Bithiah daughter of Pharaoh, who live forever with the righteous, the pious, the pure, and the upright—the doers of kindness, the masters of charity, the masters of Emunah (faith), and the masters of Torah. Not only that, but they are raised up and seated in the Heavenly Academy, and they hear Torah directly from the mouth of the Holy One, Blessed be He. For on the 17th of Tammuz, all sins are forgiven for anyone who performs Teshuvah (repentance) with a whole heart. Then all barren women are blessed with children, for then they emerge from a place above space, and all the worlds ascend to the Malchus of Atzilus—to the place where the Ark and Korach were flying in the air. Therefore, immediately after the portions of Korach, Chukkas, and Balak comes the 17th of Tammuz, which is when the rectification of the letter "Kuf" (ק) is completed—the letter that descends into Gehinnom and brings out from there the souls of the wicked who will do Teshuvah for having caused the destruction through their evil deeds, whether desecration of the Sabbath, immorality, bloodshed, or other transgressions. Because they accepted the judgment of Gehinnom with love, all their sins were atoned for, and they merit on the 17th of Tammuz to ascend and hear Torah from the mouth of the Holy One, Blessed be He, Himself. Therefore, it is said of them, "he shall surely live," for anyone who does Teshuvah on the 17th of Tammuz merits to bring out tens of thousands of souls from the judgment of Gehinnom—whether they be masters of charity, masters of Emunah, or masters of Torah—and he raises them and seats them in the Heavenly Academy, and they hear Torah from the mouth of the Holy One, Blessed be He, after he has rectified all three "Kufs" whose "feet descend to death." For the moment a person accepts the judgment of Gehinnom with love, he immediately leaves there and merits to sit in the Heavenly Academy. Gabriel and Michael go and open 48 million gates of Gehinnom and bring out from there the wicked who did sincere Teshuvah and justified the judgment of Gehinnom upon themselves. For through the sin of Adam HaRishon (the first man), the five types of the Mixed Multitude were mixed into Israel: the Nefilim, Gibborim, Anakim, Rephaim, and Amalekim. This is why we read in Parshas Chukkas about the King of Arad, who is the king of all the "Arod" (wild asses/serpents), about whom it is said, "Woe to the man who is met by an Arod, but woe to the Arod who is met by Rabbi Chanina ben Dosa." For from the bones of Balaam came all the "Arods" in the world, because he separated Rachel and Leah, and the "heels" of Leah emerged from the Keter (Crown) of Rachel. For Balaam + Balak = Rachel + Leah = Arod, Arad. Therefore, only Mashiach, who is called "Migdal Eder" (Tower of the Flock)—"And he shall come from Migdal Eder" [Targum]—will be able to rectify all the Arods, Amalekim, Nefilim, Gibborim, Rephaim, and Anakim, who are Sichon and Og and their companions, who came to the daughters of man and defiled them with their impurity. Because of them, the Holy Temple was destroyed, and Moses was not careful of them; therefore, they have no rectification, neither in the days of Mashiach nor in the Resurrection of the Dead. They descend for all generations to Sheol (the underworld) and Abaddon (destruction) and never leave there, for there is Teshuvah for everything except for the Amalekim, Anakim, Rephaim, Nefilim, and Gibborim who cast their terror in the land of the living, such as Jeroboam and his companions. Of them it is said, "brimstone and salt, a burning of the whole land," for the Holy One, Blessed be He, wanted to find merit for Jeroboam; He grabbed him by his garment and said to him, "I, you, and the son of Jesse (David) will stroll in Gan Eden (the Garden of Eden)." But Jeroboam rejected the offer of the Holy One, Blessed be He, for he was of the Nefilim, whom nothing in the world can bring back in Teshuvah. They are judged for all generations and do not succeed in clinging to Michael and Gabriel who try to bring them out of Gehinnom. For the Geulah (Redemption) will be on the 17th of Tammuz, in the secret of "for a time, times, and a half," which is the secret of Tisha B'Av and the 17th of Tammuz, about which Gabriel prophesied, "clothed in linen." Therefore, on Yom Kippur, one enters the Holy of Holies only in linen garments. Through this, we raise all the souls of the wicked that are made of flax (linen); this is why Cain offered flax, but his mistake was that he did not mix them with wool—the secret of the "ten types of melody," through which alone the Resurrection of the Dead will occur.