Esther 8:17

The Jews assembled in their cities throughout all the provinces of King Ahasuerus to send forth their hand against those who sought their calamity; and no one could stand before them, for the dread of them had fallen on all the peoples.

the Jews established and accepted a custom for themselves and for their seed and for all those who joined themselves to them, so that celebrating these two days according to what was written down and according to their fixed time from year to year would not pass away.