Charles Ellicott Commentary Esther 9:18

"But the Jews that were in Shushan assembled together on the thirteenth [day] thereof, and on the fourteenth thereof; and on the fifteenth [day] of the same they rested, and made it a day of feasting and gladness." — Esther 9:18 (ASV)

On the fifteenth day ... they rested. —Both the fourteenth and fifteenth days are now kept as the festival of Purim, the former day being the chief.