Musar su Genesi 32:8

This is the meaning of the verse, “And Mordechai knew” (Esther 4:1). That is, he knew the essence of the matter — that the decree had been issued on account of the great harm they had done to the Shechinah when they benefited from the feast of that evildoer. Therefore it states, “And Mordechai donned sackcloth and ashes and cried out with a great and bitter cry.” “And he donned sackcloth” — This was to atone for the sin and to weaken the power of the “four hundred men” who accompanied Eisav when he came to meet his brother Yaakov. For it is stated concerning this event, “And Yaakov was very afraid” (Bereishis 32:8). That is, he was afraid of the four hundred impure husks and destroyers that accompanied Eisav. This is why Mordechai donned sackcloth, because the numerical value of the word “sackcloth” [שׂק] is 400. Thus his intention was to weaken the power of the wicked Eisav. Then Mordechai informed also Esther of the great harm that had been done at the feast of Achashveirosh. Therefore it states immediately after this, “And Esther donned ‘kingship’” (Esther 5:1). That is, she donned the spirit of prophecy (Megillah 14b). Esther then proposed that they fast for three days and three nights in order to counteract the filth that Haman and Achashveirosh had injected into them by means of their feast. In this way the power of the filth that had sullied them through that eating and drinking would be enfeebled, for it was weakened through their fasting, thereby enabling the collective soul of Israel to shine as before.