Matthew Poole and continuators on Esther 9:21

21 To stablish this among them, that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same yearly:

Because both these days had been set apart this year, the latter at Shushan, the former in other parts; and because that great work of God which was the ground of this solemnity had been done both upon the 13th, and the 14th day.