Overview:

Purim is a Jewish Holy Day that celebrates the thwarting of an attempted Holocaust of the Jewish peope, which had been planned by a wicked servant named Haman. This Holocaust was thwarted by the brave actions of Queen Esther in tandem with her uncle (who was also her adopted father), Mordecai.

This salvation occurred while the Jews were in exile from Israel. At the time, Babylon had been destroyed and the Medes and Persians ruled the lands in which the Jews were exiled.  The King, Ahasuerus, had killed his wife, Vashti for not showing up to a dinner party and thus sought another queen. King Ahasuerus summoned all of the virgin young women to his palace, of which included a young Jewish woman named Hadassah (later named Esther). 

During this event, the wicked official named Haman sent out a decree to murder all of the Jews in the lands from India to Ethiopia and Shushan. On Pesach, Haman threw lots (pur) to decide the date when he would kill the Jews: 13 Adar. 

Queen Esther thwarted Haman's plan by bravely decreeing a 3 day fast among the Jewish population, then boldly entering the King's courts to reveal to the King the plans of Haman. The King was furious at Haman and decreed that Haman and his 10 sons be hung on the gallows which they had erected to hang Esther's uncle, Mordecai. 

Then King Ahasuerus sent out another decree saying the Jews were allowed to fight against the army, which Haman sent out to murder them. Purim celebrates the miraculous victory of the Jews, wherein not a single Jew was killed during the fight for their life! 

This Holy Day is celebrated with all things "hidden": costumes cover our appearances and dough covers the fillings in Hamantashen, all in commemoration of both the hidden nature of Queen Esther's Judaism and also the hidden nature of Hashem in the Megillah Esther (in which, His name is not mentioned a single time!)