Esther 8:1-17

So on that day the king Ahasuerus did give the house of Haman the Jews' enemy unto Esther the queen. [He gave the house to her.] And Mordecai came before the king; for Esther had told what he was to her (Esther 8:1).

So Esther said, "Mordecai is my cousin."

And the king took off his ring which he had taken from Haman, and he gave it to Mordecai. And Esther set Mordecai over the house of Haman. And Esther spake yet again before the king, and fell down at his feet, and besought him with tears to put away the mischief of Haman the Agagite, and the devices that he had devised against the Jews. And the king held out the golden sceptre toward Esther. So Esther arose, and stood before the king, and said, If it pleases the king, if I have found favor in your sight, and the thing seems right, and I be pleasing in your eyes, let it be written to reverse the letters devised by Haman the son of Hammedathah the Agagite, which he wrote to destroy the Jews which are in all the king's provinces: For how can I endure to see the evil that shall come to my people? how can I endure to see the destruction of my kindred? (Esther 8:2-6)

Now, there is an interesting law of the Medes and the Persians, and that is once the king has made a decree he can't change it. Once a decree has been made and sealed by the king, that decree stands. It was a law of the Medes and the Persians. You remember, Daniel got tripped up by this law of the Medes and the Persians when Darius was snookered by these guys who wanted to destroy Daniel, and they said, "Let the king make a decree that whoever prays to any other god or person for the period of thirty days will be put to death. So that the people might know how important and how wonderful you are." So the king made this proclamation, signed it, and so then Daniel went to his home, and as was his custom, the windows open towards Jerusalem, he prayed three times daily, and these guys were outside in the bushes. They ran back to the king and said, "Hey, we've got a guy who violated your proclamation." And so they brought Daniel in, and Darius and Daniel had a close relationship. And Darius sought all day some way to reverse it, but because the laws of the Medes and Persians cannot be reversed, they cannot be changed; they cannot be altered. Once it has been set you cannot reverse it.
Thus, Daniel had to be tossed into the lion's den, but Darius assured him, "Daniel, the God you serve is able to deliver you, you know." And so that same situation with the decree that Darius made, we'll get that when we get to the book of Daniel.
Now, he can't change the decree. However, he let Mordecai make another decree that on the thirteenth day of March the Jews would be able to defend themselves from against those that would seek to kill them. And so, they could also take vengeance on those who were seeking to destroy them. So he gave them the right to defend themselves and also the right to destroy any who would seek to destroy them. "