Esther 9:17-21

This was done on the thirteenth day of the month Adar, and on the fourteenth day they rested and made it a day of feasting and rejoicing.

But the Jews who were in Susa assembled on the thirteenth and the fourteenth of the same month, and they rested on the fifteenth day and made it a day of feasting and rejoicing.

Therefore the Jews of the rural areas, who live in the rural towns, make the fourteenth day of the month Adar a holiday for rejoicing and feasting and sending portions of food to one another. The Feast of Purim Instituted

Then Mordecai recorded these events, and he sent letters to all the Jews who were in all the provinces of King Ahasuerus, both near and far,

obliging them to celebrate the fourteenth day of the month Adar, and the fifteenth day of the same month, annually,