The book of Esther was written to "document the origins of the Jewish observance of Purim, which celebrates Israel's survival and God's faithfulness." (ESV)
It says in the book itself that "the Jews firmly obligated themselves and their offspring and all who joined them, that without fail they would keep these two days according to what was written and at the time appointed every year... throughout every generation... Queen Esther.. and Mordecai the Jew gave full written authority, confirming this..." Esther 9:20-32
It is recorded in the Book of Esther that when the enemy of the Jews: Haman, a Persian official, in 480 B.C. tried to wipe out all the Jews of the provinces of Persia on the 13th of the Jewish calendar month of Adar (which this year, 2012, was March 7th) it backfired on him and the reverse happened on that same appointed day.
Haman was an evil man full of selfishness, pride and envy. Because Mordecai would not give him respect and honor that he assumed he deserved he plotted to kill him and all his people, the Jews.
(Esther 1-3)
Right before this, Esther, a Jew, Mordecai's cousin, found favor in the King of Persia's eyes: King Ahasuerus, who reigned from India to Ethiopia, during this time. She became queen, and was used "for such a time as this" (Esther 4:14). At the risk of her life, she went before the king and pleaded for the lives of Mordecai and all the Jews, and it ended in victory.
This is why Jews celebrate Purim every year, with one day of fasting (commemorating the 3 days of fasting and prayer Esther and all the Jews underwent before presenting herself before the king). This is followed by one 1 to 2 days of celebration commemorating the victories of the Jews that followed. (Esther 7-9)
This is another example of how the Jews had victory over their enemies, that the provinces even feared them (Esther 9:3). Another example of how the wicked will fail (Proverbs 11:2). Also, an excellent example of how one woman can have a tremendous effect on multitude of people and how Esther's boldness was commended (Esther 4:16). Praise God Adonai!