In 473 BC during the reign of Xerxes I of Persia [or Ahasuerus of Iran], Haman the Amalekite was the senior advisor to the king. Exodus 17 and 1 Samuel 15 describes the Amalekites' ancient feud with Israel that leads to Haman's plotted to kill the Jews and cast purim or lots that chose the 14th of Adar (February/March) by chance. However, God put Queen Esther and Mordecai in place for such a time as this. Haman was hanged, his sons were killed and the Amalekites were judged for persecuting the Jews. The Jewish people were saved from harm and Mordecai was appointed top advisor to the king. A day of mourning became a day of deliverance.
Haman sought to destroy all the Jews that were throughout the whole kingdom of Ahasuerus, even the people of Mordecai. In the first month, that is, the month Nisan, in the twelfth year of king Ahasuerus, they cast Pur, that is, the lot, before Haman from day to day, and from month to month, to the twelfth month, that is, the month Adar (Esther 3:6, 7).
Purim is not part of the original seven appointed days of Leviticus 23 but has been a holiday for 2500 years. People wear costumes of the main characters in the story. The book of Esther is read and every time that Haman's name is mentioned, the people yell and use noise makers to blot out his name. A part of the celebration is eating a festive meal and giving to the poor.
[Mordecai established] this among them, that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly, As the days wherein the Jews rested from their enemies, and the month which was turned unto them from sorrow to joy, and from mourning into a good day: that they should make them days of feasting and joy, and of sending portions one to another, and gifts to the poor (Esther 9:21-22).