Albert Barnes Commentary Esther 9:31

"to confirm these days of Purim in their appointed times, according as Mordecai the Jew and Esther the queen had enjoined them, and as they had ordained for themselves and for their seed, in the matter of the fastings and their cry." — Esther 9:31 (ASV)

The matters of the fastings and their cry - The Jews of the provinces had added to the form of commemoration proposed by Mordecai certain observances with respect to fasting and wailing, and Mordecai’s second letter sanctioned these.