Purim commemorates the deliverance of the Jewish people in the ancient Persian Empire from destruction in the wake of a plot by Haman, a story recorded in the Book of Esther (Megillat Esther).
According to the Book of Esther, Haman, royal vizier to King Ahasuerus (presumed to be Xerxes I of Persia), planned to kill all the Jews in the empire, but his plans were foiled by Mordecai and his adopted daughter Queen Esther. The day of deliverance became a day of feasting and rejoicing.
Purim is celebrated by giving mutual gifts of food and drink (mishloach manot), giving charity to the poor (mattanot la-evyonim), a celebratory meal (se'udat Purim), public recitation of the Scroll of Esther (keriat ha-megillah), and additions to the prayers and the grace after meals (al hannisim). Other customs include drinking wine, wearing of masks and costumes, and public celebration.
Purim is celebrated on the 14th day of of Adar (Adar II in leap years), the day following the victory of the Jews over their enemies. In earlier times Purim was celebrated on the 15th of the month in cities that were protected by a surrounding wall at the time of Joshua. This was known as Shushan Purim. Today, only Jerusalem celebrates Purim on the 15th.