Esther 9:21 - Joseph Benson’s Commentary on the Old and New Testaments

To establish this to keep the fourteenth day, and the fifteenth day of the month Adar The twelfth month; just a month before the passover. Thus the first and the last month of the year were kept in remembrance of the days when God preserved them. They were enjoined to keep two days together, because both those days had been set apart this year, the latter at Shushan, and the former in other parts; and because that victory over their enemies, which was to be the ground of this festival, had been achieved on both the thirteenth and fourteenth days. Let us not be niggardly in our returns of praise to him, who bestows his favours so liberally upon us.