ז בַּחֹדֶשׁ הָרִאשׁוֹן הוּא–חֹדֶשׁ נִיסָן בִּשְׁנַת שְׁתֵּים עֶשְׂרֵה לַמֶּלֶךְ אֲחַשְׁוֵרוֹשׁ הִפִּיל פּוּר הוּא הַגּוֹרָל לִפְנֵי הָמָן מִיּוֹם ׀ לְיוֹם וּמֵחֹדֶשׁ לְחֹדֶשׁ שְׁנֵים–עָשָׂר הוּא–חֹדֶשׁ אֲדָר
7. In the first month – it is the month of Nissan – in the twelfth year of the King Achashverosh, he cast a “pur” – it is a lottery – before Haman, from day to day and from month to the twelfth month – it is Adar.
Both the Ibn Ezra and Malbim write that this event occurring in Nissan to give the Jews the opportunity to do teshuva. The only reason the Jews were not killed is because of the deep repentance they performed for the sin of disobeying Mordechai by attending Achashverosh’s party. Indeed, the teshuva was complete when all of the Jews – who had previously ignored Mordechai’s pleas – obeyed his command to fast. Mothers could only keep food from their children for three days by submitting their will to the will of the sages.