“Haman the Agagite son of Hammedatha, the enemy of all the Jews, had plotted against the Jews to destroy them. He had cast Pur that is ‘the lot’ to crush and destroy them. When Queen Esther came before the king, he gave orders in writing that the wicked plot that Haman had devised against the Jews should come upon his own head. He and his sons should be hanged on the gallows. Therefore they called these days Purim, after the word Pur. All of this was written in this letter. Because of what they had faced in this matter, and of what had befallen them, the Jews established and accepted as a custom for themselves and their descendants, and all who joined them. Without fail they would continue to observe these two days every year, as it was written, and at the time appointed. These days should be remembered and kept throughout every generation, in every family, province, and city. These days of Purim should never fall into disuse among the Jews. The commemoration of these days should never cease among their descendants.”
This is an official explanation of the feast of Purim. Since this does not have Torah approval, there is a strong emphasis on written documents. After the captivity and exile there is a great insistence on written documents. Purism comes from the idea of casting lots, which Haman did, to decide on what day the persecution and the destruction of the Jews should take place. The movement of Pur into Purim is simple enough. Interesting enough, the Greek text continued to call Haman a Macedonian rather than an Agagite. In the Greek text there is the explicit information that Pur means casting lots in Persian. The Greek text insists that Mordecai established this feast.