Tonight is Purim. Happy Purim from UJGS!
What is Purim? It is the Jewish holiday that celebrates the story told in the Book of Esther.
In Shushan, Persia, Esther was raised by her cousin, Mordechai. King Ahasuerus, after dismissing his wife, went looking for a new wife, choosing Esther, without realizing she was Jewish.
The villain of the story is Haman, the king's advisor, who plotted to kill all of the Jewish people when Mordechai refused to bow down to him.
Esther fasted for three days then approached the king, finally telling him that she was Jewish, and asked him to save the lives of her people. She prevailed and Haman was hanged on the gallows that he had prepared by him for Mordechai.
Purim is celebrated on the 14th of Adar, except in Jerusalem and other cities that were walled during the time of Joshua, where it is celebrated on the 15th. In a leap year, it occurs during Adar II, the leap month.
Purim is a festive holiday which includes dressing up in costumes, eating Hamentashen, and listening to a reading of the Megillah (the Book of Esther).
More details about the holiday can be found at Judaism 101.