On the thirteenth day - Haman had, apparently (compare Esther 3:7 with Esther 3:13), obtained by his use of the lot the 13th day of Adar as the lucky day for destroying the Jews. This may have caused him to fix on the 13th day of another month for the commencement of his enterprise. So, the Jews throughout the empire had from 9 to 11 months of warning of the peril which threatened them.
Related Commentaries of Esther 3:12
Esther 3:12
12 Then were the king's scribesb called on the thirteenth day of the first month, and there was written according to all that Haman had commanded unto the king's lieutenants, and to the governors that were over every province, and to the rulers of every people of every province according to the writing thereof, and to every people after their language; in the name of king Ahasuerus was it written, and sealed with the king's ring.