If we paid close attention to the Megillah reading, we would notice the number of years it took for the whole Purim story to unfold.
Achashveirosh's party was in the third year of his reign. Esther became queen in the seventh year of his reign, and Haman was finally hanged in the twelfth year of his reign.
Many don't realize that the problems of the Jewish people did not end with the hanging of Haman. This took place on Pesach but there was still an edict that went out giving permission to kill all the Jews on the thirteenth of Adar.
Esther pleaded with Achashveirosh that something needed to be done to stop this decree. It was at the end Sivan, more than two months later that anew decree went out. In this executive order, it was the Jews who were given permission to take revenge against their enemies.
75,000 were killed in the rest of the kingdom on the thirteenth, and they celebrated on the fourteenth. Esther asked for an extra day in Shushan to eliminate the Jew haters there. In Shushan, they fought on the thirteenth and the fourteenth, and celebrated on the fifteenth. Hence, Shushan Purim.