Mordecai ordered the feast to be celebrated on the same days of its first, spontaneous celebration: the fourteenth and fifteenth days of Adar. Esther 9:19 and Esther 9:23 suggest that the Jews in Susa only celebrated it on the fifteenth, while the rest of the Jews began their celebrations on the fourteenth. However, by the time of the historian Flavius Josephus (around AD 70), all Jews would be celebrating the Purim feast on both days.
21 obliging them to keep the fourteenth day of the month Adar and also the fifteenth day of the same, year by year,