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, They called these days Purim - That is from pari, the lot; because, as we have seen, Haman cast lots to find what month, and what day of the month, would be the most favorable for the accomplishment of his bloody designs against the Jews. See on Esther 3:7 (note).
And of that which they had seen - The first letter to which this second refers, must be that sent by Mordecai himself. See Esther 9:20.