Purim is one of the most joyous and fun holidays on the Jewish calendar. It commemorates a time when the Jewish people living in Persia were saved from extermination.
The story of Purim is told in the Biblical book of Esther. The heroes of the story are Esther, a beautiful young Jewish woman living in Persia, and her cousin Mordecai, who raised her as if she were his daughter. Esther was taken to the house of Ahasuerus, King of Persia, to become part of his harem, and he loved her more than his other women and made her queen. But the king did not know that Esther was a Jew, because Mordecai told her not to reveal her nationality.
The villain of the story is Haman, an arrogant, egotistical advisor to the king. Haman hated Mordecai because Mordecai refused to bow down to Haman, so Haman plotted to destroy the Jewish people. In a speech that is all too familiar to Jews, Haman told the king, “There is a certain people scattered abroad and dispersed among the peoples in all the provinces of thy kingdom; and their laws are diverse from those of every people; neither keep they the king's laws; therefore it does not profit the king to suffer them.” Esther 3:8. The king gave the fate of the Jewish people to Haman, to do as he pleased to them. Haman planned to exterminate all of the Jews.
Mordecai persuaded Esther to speak to the king on behalf of the Jewish people. This was a dangerous thing for Esther to do, because anyone who came into the king's presence without being summoned could be put to death, and she had not been summoned. Esther fasted for three days to prepare herself, then went into the king. He welcomed her. Later, she told him of Haman's plot against her people. The Jewish people were saved, and Haman was hanged on the gallows that had been prepared for Mordecai.
The book of Esther is unusual in that it is the only book of the bible that does not contain the name of G-d. In fact, it includes virtually no reference to G-d. Mordecai makes a vague reference to the fact that the Jews will be saved by someone else, if not by Esther, but that it the closest the book comes to mentioning G-d. Thus, one important message that can be gained from the story is that G-d often works in ways that are not apparent, in ways that appear to be chance, coincidence or ordinary good luck.
Purim is celebrated on the 14th day of Adar, which is usually in March. The 14th of Adar is the day that Haman chose for the extermination of the Jews. In leap years, when there are two months of Adar, Purim is celebrated in the second month of Adar, so it is always one month before Passover. In cities that were walled in the time of Joshua, Purim is celebrated on the 15th of the month, because the book of Esther says that in Shushan (a walled city), deliverance from the massacre was not complete until the next day.
The word “Purim” means “lots” and refers to the lottery that Haman used to choose the date for the massacre.
The Purim holiday is preceded by a minor fast, the Fast of Esther, which commemorates Esther's three days of fasting in preparation for her meeting with the king.
The primary commandment related to Purim is to hear the reading of the book of Esther. The book of Esther is commonly known as the Megillah, which means scroll. Although there are five books of Jewish scripture that are properly referred to as megillahs (Esther, Ruth, Ecclesiastes, Song of Songs, and Lamentations), this is the one people usually mean when the speak of The Megillah. It is customary to boo, hiss, stamp feet and rattle gragers (noisemakers; see illustration) whenever the name of Haman is mentioned in the service. The purpose of this custom is to “blot out the name of Haman.”
We are also commanded to eat, drink and be merry. According to the Talmud, a person is required to drink until he cannot tell the difference between “cursed be Haman” and “blessed be Mordecai,” though opinions differ as to exactly how drunk that is.
In addition, we are commanded to send out gifts of food or drink, and to make gifts to charity. The sending of gifts of food and drink is referred to as shalach manos (lit. sending out portions). Among Ashkenazic Jews, a common treat at this time of year is hamentaschen (lit. Haman's pockets). These triangular fruit-filled cookies are supposed to represent Haman's three-cornered hat.
It is customary to hold carnival-like celebrations on Purim, to perform plays and parodies, and to hold beauty contests. I have heard that the usual prohibitions against cross-dressing are lifted during this holiday, but I am not certain about that. Americans sometimes refer to Purim as the Jewish Mardi Gras.
Work is permitted as usual on Purim, unless of course it falls on a Saturday.
Sources: Judaism 101