Purim is the Jewish holiday that celebrates the deliverance of the Jewish people from the intended genocide planned by Haman— the story is described in the book of Esther.
“Haman said to king Ahasuerus, There is a certain people scattered abroad and dispersed among the peoples in all the provinces of your kingdom; and their laws are diverse from those of every people; neither keep they the king’s laws: therefore it is not for the king’s profit to allow them. If it please the king, let it be written that they be destroyed: and I will pay ten thousand talents of silver into the hands of those who have the charge of the king’s business, to bring it into the king’s treasuries. The king took his ring from his hand, and gave it to Haman the son of Hammedatha the Agagite, the Jews’ enemy. The king said to Haman, The silver is given to you, the people also, to do with them as it seems good to you.” (Esther 3:8–11 WEB)
I won't repeat the story of the deliverance here— the book of Esther tells it in detail. But I will quote the parts of the final chapter that describe Purim.
“Now in the twelfth month, which is the month Adar, on the thirteenth day of the same, when the king’s commandment and his decree drew near to be put in execution, on the day that the enemies of the Jews hoped to have rule over them, (whereas it was turned to the contrary, that the Jews had rule over those who hated them,) the Jews gathered themselves together in their cities throughout all the provinces of the king Ahasuerus, to lay hand on such as sought their hurt: and no man could withstand them; for the fear of them was fallen on all the peoples.” (Esther 9:1–2 WEB)
“Therefore do the Jews of the villages, who dwell in the unwalled towns, make the fourteenth day of the month Adar a day of gladness and feasting, and a good day, and of sending portions one to another. Mordecai wrote these things, and sent letters to all the Jews who were in all the provinces of the king Ahasuerus, both near and far, to enjoin those who they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly, as the days in which the Jews had rest from their enemies, and the month which was turned to them from sorrow to gladness, and from mourning into a good day; that they should make them days of feasting and gladness, and of sending portions one to another, and gifts to the needy. The Jews undertook to do as they had begun, and as Mordecai had written to them; because Haman the son of Hammedatha, the Agagite, the enemy of all the Jews, had plotted against the Jews to destroy them, and had cast Pur, that is the lot, to consume them, and to destroy them; but when the matter came before the king, he commanded by letters that his wicked device, which he had devised against the Jews, should return on his own head, and that he and his sons should be hanged on the gallows. Therefore they called these days Purim, after the name of Pur. Therefore because of all the words of this letter, and of that which they had seen concerning this matter, and that which had come to them, the Jews ordained, and took on them, and on their seed, and on all such as joined themselves to them, so that it should not fail, that they would keep these two days according to the writing of it, and according to the appointed time of it, every year; and that these days should be remembered and kept throughout every generation, every family, every province, and every city; and that these days of Purim should not fail from among the Jews, nor the memory of them perish from their seed.” (Esther 9:19–28 WEB)