Shushan Purim is a unique day in the cycle of Jewish holidays. Purim is the only holiday whose date depends on where you happen to celebrate it. For most of the world, Purim occurs on the 14th of Adar. However, if you happen to reside in Jerusalem or the city of Shushan (where the story of Purim took place) Purim is on the 15th of Adar. Therefore, the observance is called "Shushan Purim."