For this reason these days are called Purim, from the word Pur. Because of all the instructions in this letter as well as what they had witnessed and what had happened to them,
the Jews bound themselves, their descendants, and all who joined with them [to a commitment] that they would not fail to celebrate these two days each and every year according to the written instructions and according to the time appointed.
These days are remembered and celebrated by every generation, family, province, and city, so that these days of Purim will not lose their significance in Jewish life and their memory will not fade from their descendants.
References for Esther 9:28
Queen Esther daughter of Abihail, along with Mordecai the Jew, wrote this second letter with full authority to confirm the letter about Purim.
References for Esther 9:29
He sent letters with messages of peace and faithfulness to all the Jews who were in the 127 provinces of the kingdom of Ahasuerus,
References for Esther 9:30
in order to confirm these days of Purim at their proper time just as Mordecai the Jew and Queen Esther had established them and just as they had committed themselves and their descendants to the practices of fasting and lamentation.
So Esther's command confirmed these customs of Purim, which were then written into the record.