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Repentance of the Jews: 
Mordechai knew of the decree and proceeded to tear his clothing and wear sackcloth, walking the streets of Shushan shouting a bitter cry. Word of the evil decree reached the Jews of every province and they entered into a state of mourning, grief and lamentation.
How did Mordechai discover the evil decree?
He was told in a dream by Eliyahu Hanavi. The Midrash states that Eliyahu went to Moshe Rabbeinu to help stop the decree and Moshe asked him “Is there any Kosher man still left in their generation”. Eliyahu replied that Mordechai is a righteous man. Moshe said “Go tell him to pray for the Jews below and I will pray for them above and ask for mercy from G-d.” Eliyahu immediately went to tell Mordechai of the decree.
Was Mordechai taken by surprise by the decree?
The verse states that “And Mordechai knew all that was done”. The Midrash states that Mordechai actually dreamt of the decree against the Jews and its miraculous outcome in the second year of reign of Achashveirosh. The dream was very cryptic, with parables that related to the events. When Mordechai heard of the decree he immediately understood that this was in fulfillment of his prophetic dream. He related this dream to Esther. Alternatively Mordechai already knew that the Jews were decreed to be annihilated at the times of Nebuchadnezzar when they prostrated themselves to the idols.
Why did Mordechai scream in prayer?
Certainly Mordechai was aware that G-d can hear even whispers, as did Chanah which is the mother and source of prayer. Rather this scream had an inner intent towards Yitzchak Avinu. Mordechai was saying “Yitzchak why did you listen to the scream of Eisav and give him the blessings. Due to these blessings we are now suffering threat of annihilation.”
Mordechai meets the three children:
Mordechai met three Jewish children after he discovered the decree and asked them to tell him a verse from scripture. One child answered “Al Tira Mipachad Pisom”. A second child answered “Utzu Eitzah Vesufar”. The third child answered “Vead Zikna Ani Hu”. When Mordechai heard this he rejoiced. The reason for his rejoicing is because the random saying of versus by children is considered like a prophecy. The prophecy was that the Jewish people will strengthen their faith in Hashem, and hence annul the decree that was made due to their lack of faith expressed in them bowing to the idols.
What actions did Mordechai take to counteract the decree?
As soon as Mordechai was informed of the decree he gathered a number of children and had them fast and wear sackcloth.
 See Esther Raba 18
 Yalkut Shimoni 1056; Esther Raba 28
 Yalkut Shimoni 1056
 Midrash Esther Raba 18 [7/17]; Yalkut Shimoni 1057
 Likkutei Sichos 21 p. 206 based on Smag Lo Saseh 51; See Gitin 6a; Chulin 95b; Rama 179/4 and Shach 179/5; Kitzur SHU”A 166/2
 Likkutei Sichos 21 p. 206 as these verses that were said by the children represents words of faith.
 Esther Raba 30