Purim is a great holiday we celebrate on the 14th of Adar. The story about Purim with the salvation of the Jewish people in Persian Empire of Haman, to plot to kill all Jewish people, men, women, the elderly and children every single day.
The story begins when King Ahasuerus had his wife Queen Vashti executed for not fallowing his orders, he then arranged a beauty pageant to hind him a new Queen. A Jewish girl named Esther was in favor and she became the new Queen, without revealing her true identity.
In the meantime, Haman, a Jew-hater was appointed to be Prime Minister of the Persian Empire.
Mordechai, the leader of the Jewish people and also cousin of Esther, was defiant of the Kings orders and refuse to bow down to Haman. Haman was enraged and convinced the King to order a decree of extermination of all Jews on the 13th of Adar, this was the date chosen by lottery made by Haman.
Mordechai convinced all Jews to repent, fast and pray to HaShem. Meanwhile Esther asked the King and Haman to join her to a party.
At this party, Esther revealed her true identity. Haman was hanged and Mordechai was made the new Prime Minister and he issued a new decreed, granting the Jews to defend themselves against their enemies.
On Adar 13, the Jews rallied together and killed many of their enemies, it took one more day to finish the job.
On the 14th of Adar they celebrated their victory, this was in the Capital of Shushan
While the people outside Shushan celebrated their victory on the Adar 14, the Jews in Shushan were still very much ingaged in the task to freed the City of it murderes and other villians, it was only the fallowing day that they also could celebrate this great salvation.
From this time on, all Jews celebrate on the 14th of Adar and feel obligated to observe every year the fast of Esther on the 13th of Adar, the day before Purim and to mark this day, all Jews fallow their repentance and The Megillah is read every year on the eve of Purim in Shul