Esther 9:28-32

So these days were to be remembered and celebrated throughout every generation, every family, every province, and every city; and these days of Purim were not to be neglected by the Jews, or their memory fade from their descendants.

Then Queen Esther, daughter of Abihail, with Mordecai the Jew, wrote with full authority to confirm this second letter about Purim.

He sent letters to all the Jews, to the 127 provinces of the kingdom of Ahasuerus, namely, words of peace and truth,

to establish these days of Purim at their appointed times, just as Mordecai the Jew and Queen Esther had established for them, and just as they had established for themselves and for their descendants, with instructions for their times of fasting and their mourning.

The command of Esther established these customs for Purim, and it was written in the book.