never ever trust those (((bastards)))

In the narrative, King Ahasuerus is drunk at a festival and orders his queen, Vashti, to appear before him and his guests to display her beauty. When she refuses to come, he deposes her and seeks a new queen through a sort of beauty pageant. Esther, an orphan daughter of a Benjamite named Abihail and under the protection of her cousin Mordecai, who are members of the Jewish exile community in Persia, is chosen.

Later, Mordecai refuses to bow to Haman the Agagite, who was recently made Ahasuerus' highest advisor, and Haman requests and is given permission to order all the Jews in Persia to be killed. When Esther learns of this, Mordecai tells her to reveal to the king that she is Jewish and to ask him to repeal the order. Esther hesitates, saying that she could be put to death if she goes to the king without being summoned and the king does not want to see her; Mordecai urges her to try. She goes to the king, and the king welcomes her, and says he will give her anything she wants. Instead of asking directly about the order, she invites the king and Haman to a banquet the next day. During the banquet, the king again asked Esther if there is anything she wants, and this time she asks the king to spare her life and that of all of the Jews. The king asks who was threatening them, and she names Haman. Haman throws himself at her feet; the king thinks that Haman is attacking her and orders him to be put to death, and gives all Haman's possessions to Esther. Esther tells the king about Mordecai's role in her life, and the king makes Mordecai his highest advisor. Esther then asks the king to revoke the order, and king allows Esther and Mordecai to do so, however they wish. They send out an order in the king's name that Jews can assemble and defend themselves, and can kill anyone who threatens them, and their families, and take their goods. On the thirteenth day of Adar, the same day that Haman had set for them to be killed, the Jews do so in one city, killing 500 people but not taking plunder, and they kill around 75,000 the next day again not taking plunder, and then they feast.[8]

tl;dr: Purim is the celebration of a Jewish scheme where they succeeded in undermining their host nation (Persia).