Shushan Purim

In certain ancient walled cities – Jerusalem is the primary example – Purim is observed not on the 14th of Adar, the date of its observance everywhere else, but on the 15th of Adar. This is to commemorate that fact that in the ancient walled city of Shushan, where the battles between the Jews and their enemies extended for an additional day, the original Purim celebration was held not on the 14th but on the 15th of Adar. See Walled Cities for more on this.

The 15th of Adar is thus called "Shushan Purim" and is a day of joy and celebration also in those places where it is not observed as the actual Purim.