Charles Ellicott Commentary Esther 9:21

"to enjoin them that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly," — Esther 9:21 (ASV)

And the fifteenth day of the same. —The Jews in the provinces had already made the fourteenth day a day of gladness and feasting. Mordecai now commands that the fifteenth also be observed in this manner.