Purim is the only feast found in the Old Testament that Jews annually celebrate which is not legislated by Mosaic law. The popular feast of Hanukkah is not found in the Old Testament, as it commemorates events that occurred more than two hundred years after the last Old Testament book was written around 400 B.C.
The reasons why the Jews in Persia were threatened with slaughter and how Purim came to be considered a yearly period of rejoicing is found in the book of Esther.
A man named Haman, who was ambitious and egotistical, is prime minister of Persia under King Xeres (Esther 3:1). He becomes enraged when Mordecai, a Jewish official in the royal court, refuses in good conscience to bow before him and revere him as a god (verses 2 to 5).
Seeking to destroy not only Mordecai but all the Jews in the Kingdom, Haman casts lots (the Hebrew word for 'lots' is Pur or Purim, Strong's Concordance #H6332, from where the feast gets its name) to determine which day to begin carrying out his plot. He then persuades King Xeres (Ahasuerus) through lies and offering 10,000 talents of silver to the King's treasury (worth around $219 million dollars today) to issue an edict ordering all Jews be killed.
8. And Haman said to King Ahasuerus, "There is a certain people scattered abroad and dispersed among the people, in all the provinces of your kingdom. And their laws are different from those of all other people, neither do they keep the king's laws. Therefore it is not in the king's interest to allow them to live. 9. If it pleases the king, let it be written that they may be destroyed . . ."
10. And the king took his ring from his hand, and gave it to Haman the son of Hammedatha the Agagite, the Jews' enemy. 11. And the king said to Haman, "The silver is given to you, the people also, to do with them as seems good to you" (Esther 3:8 - 11, HBFV)
Haman's plot is ultimately thwarted through the efforts of Mordecai and Queen Esther (who were cousins). Not only is he hanged on the same gallows he planned to use for Mordecai (Esther 7:10), his ten sons are also hung as well (9:13 -14).
A tomb believed by some to hold the remains of Esther and her cousin Mordecai is located in Hamadan, a city in the Northwest part of modern day Iran. Ironically, even though the book of Esther delineates the Eternal's miraculous hand at saving the entire Jewish nation from slaughter, the word 'God' is not found in the book.
Purim begins at sunset on March 23 in 2016, sunset March 11 in 2017 and sunset February 28 in 2018.