So what's the Feast of Lots or Purim? The root word in Purim is pur, which means "lots." Haman was the Prime Minister of Persia at that time. He cast lots to see what day they would kill the Jews and the lot fell on Adar 13. God used Esther to intervene and He saved the Jews from genocide.
Purim is also known as 'The Feast of Esther', 'The Feast of Lots', and 'Mordecai's Day.' It's called the 'Feast of Esther' in remembrance of Queen Esther's banquet with the King. Its called 'The Feast of Lots' because Haman cast lots to determine the day the Persians would kill all Jews. It was called 'Mordecai's Day' in ancient times. Mordecai was the uncle of Esther, he convinced Esther to intervene with the King to save the Jews.