purim, the change of command and the dates

 esther 8.7 Then the king Ahasuerus said unto Esther the queen and to Mordecai the Jew, Behold, I have given Esther the house of Haman,

Esther 9King James Version (KJV)
9 Now in the twelfth month, that is, the month Adar, on the thirteenth day of the same, when the king's commandment and his decree drew near to be put in execution, in the day that the enemies of the Jews hoped to have power over them, (though it was turned to the contrary, that the Jews had rule over them that hated them;)

2 The Jews gathered themselves together in their cities throughout all the provinces of the king Ahasuerus, to lay hand on such as sought their hurt: and no man could withstand them; for the fear of them fell upon all people.
3 And all the rulers of the provinces, and the lieutenants, and the deputies, and officers of the king, helped the Jews; because the fear of Mordecai fell upon them

  Esther 9.17:On the thirteenth day of the month Adar; and on the fourteenth day of the same rested they, and made it a day of feasting and gladness. 

  Esther 9:21   To stablish this among them, that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly,

  Esther 9.26:Wherefore they called these days Purim after the name of Pur. Therefore for all the words of this letter, and of that which they had seen concerning this matter, and which had come unto them,