Purim is one of the most joyous and festive holidays on the Jewish calendar and its celebration includes beauty contests, feasting, singing, gift giving, and the performance of plays and parodies. A Carnival-like atmosphere is traditional, and the holiday is referred to by some as the Jewish Mardi Gras.
Purim's story comes from The Scroll of Esther, referred to as "The Megillah" (or simply, "scroll"). It recounts the conflict between Haman, powerful advisor to the King of Persia, and the Jewish People during the second exile. Esther was the second wife of Achashveirosh, the King of Persia. Her uncle, and surrogate father, Mordechai had counseled Esther to hide her Jewish ancestry from the king when she became part of his harem.
Achashveirosh was not entirely popular with his subjects. Mordechai intercepted a plot to assassinate the king and informed Esther, who then passed the news on to the king making sure that he knew that it was Mordechai who had uncovered this treachery. The would-be assassins were hanged, and the incident was promptly forgotten!
When the King later appointed a new right-hand man named Haman, he commanded all his subjects to bow down and prostrate themselves before the new Official. Mordechai however, refused. Haman was enraged by this and resolved that all the Jewish people would pay for the insult. Haman persuaded the King to decree that the Jews be exterminated for their defiance and unusual customs and cast lots to determine the most auspicious date for the intended massacre ("Purim" means "lots", as in a lottery). He then constructed a gallows specifically intended for the execution of Mordechai.
Unbeknownst to Haman, who had planned to ask for the death of Mordechai, the King was reminded of the Jew's earlier assistance and inquired of his villainous advisor (without telling him who was being referred to) how to properly reward an honorable and loyal citizen. Assuming that he himself was the subject of the discussion, Haman suggested that the person be dressed in the King's robes and crown, seated on the King's horse and paraded through the city.
"Then hurry", said the king "Get the robe and the horse and do as you have suggested for Mordechai the Jew."
That evening, Esther used her influence to reveal Haman's treachery to the King, and the Advisor was hung on the gallows that had been intended for Mordechai.
Esther begged the king to avert the sentence of genocide that had been engineered by Haman. But while the king could not revoke a signed and sealed decree, he let Esther and Mordechai author a new one allowing that the Jews might prepare and defend themselves against all who would attack or threaten them. The Jews resisted and fought off their attackers for two days - all of Haman's 10 sons were killed.
The primary observance of Purim involves the reading of the Megillah, or book of Esther. This book is unusual in that it is the only book of the Bible that does not contain the name of God. In fact, it includes virtually no reference to God. Because of this, and its underlying themes of subterfuge and disguise, the wearing of masks and costumes has become part of the celebrations.