Esther 9:23-27

So the Jews undertook what they had started to do, and what Mordecai had written to them.

For Haman the son of Hammedatha, the Agagite, the adversary of all the Jews, had schemed against the Jews to eliminate them, and had cast Pur, that is the lot, to disturb them and eliminate them.

But when it came to the king’s attention, he commanded by letter that his wicked scheme which he had devised against the Jews was to return on his own head, and that he and his sons were to be hanged on the wooden gallows.

Therefore they called these days Purim after the name of Pur. And because of the instructions in this letter, both what they had seen in this regard and what had happened to them,

the Jews established and made a custom for themselves, their descendants, and for all those who allied themselves with them, so that they would not fail to celebrate these two days according to their regulation and according to their appointed time annually.