Now, on the day when the Jews were going to be massacred (which, for the record, was the thirteenth day of the twelfth month, called "Adar"), the Jews end up banding together and killing the people who originally were going to try to kill them.
All the governors and officials and satraps support the Jews and stand out of their way, because they're afraid of Mordecai, who has now become extremely powerful.
The Jews kill over five hundred people in the citadel of Susa and kill every one of Haman's ten sons—but they don't plunder their wealth after they kill them.
The king hears about this news and asks Esther what else she'd like him to do. She says that the Jews should be allowed to keep killing their enemies for another day and that Haman's ten sons' corpses should be hanged from the gallows.
The king agrees. The Jews kill three hundred more people and hang Haman's sons' corpses, while still refraining from plunder.
At the same time, throughout all the provinces, the Jews have killed over seventy-five thousand of their enemies. They then rest and celebrate on the following day.
The people in the provinces rest on the fourteenth and the people of Susa rest on the fifteenth of the month, celebrating at slightly different times.
The Purpose of Purim
This is how the holiday of Purim came to exist: Mordecai sends out another decree telling everyone that they should observe the fourteenth and fifteenth days of Adar as a holiday every year from now on.
To celebrate how their near demise was turned into a day of rejoicing (not to mention successful revenge), they should give food gifts to one another and give presents to the poor, feasting and making merry.
The book explains that the reason "Purim" has its name is because the word "Pur" means "the lot" (like in gambling), since Haman cast lots to determine the day he would destroy the Jews. But instead, the lot was reversed against him—he determined the days of his own destruction.
Purim became a big deal says the book itself (and it did). The book says that every Jew should continue to observe the holiday on and on through history.
Esther and Mordecai both fix Purim as an official holiday for the Jews with their decree, along with regulating their times of fasting and lamentations.