Held on the day after Purim, "Shushan Purim" is when residents of Jerusalem and other ancient walled cities celebrate this joyous holiday. Here's why:

The Delayed Celebration

The battles fought between the Jews and their enemies throughout the Persian empire took place on 13 Adar. Around the world, the Jews rested and celebrated on the following day—14 Adar. In the capital city of Shushan, however, where there were a greater number of Jew-haters, the fighting continued for two days, 13 and 14 Adar. The victory celebrations in Shushan were thus held on the 15th.

When the holiday of Purim was set for the 14th of Adar, the sages instituted that Shushan residents perpetually observe Purim on the 15th of Adar—the day when the Shushanite Jews celebrated. The 15th of Adar is hence known as “Shushan Purim.”

Which Cities Are Included?

Along with Shushan (which is located in modern-day southwestern Iran), all cities that were walled at the time when the Israelites, under the leadership of Joshua, entered Canaan, observe Purim on the 15th.1

Today, the only city that we are certain had walls in Joshua’s times is JerusalemToday, the only city that we are certain had walls in Joshua’s times is Jerusalem.2 And indeed, in the holy city, Purim is festively celebrated one day after all other cities.3 There are a number of other ancient cities in Israel, such as Jaffa and Tiberias, regarding which there is a reasonable doubt whether they were walled in Joshua’s times. These cities observe two days of Purim.

Shabbat and the Three-Day Purim

The 14th of Adar—the “regular Purim”—can never fall on Shabbat, but the 15th of Adar can. For the residents of Jerusalem, this results in the unique phenomenon of a “triple” or three-day Purim. Because a number of the Purim mitzvot cannot be performed on Shabbat, the observances are spread over a period of three days.

Friday:

Megillah reading,4 night and day. Gifts to the poor during the daylight hours of Friday.5

Shabbat:

The Purim VeAl HaNissim is added to all the day’s prayers and Grace After Meals. The maftir of the day is the Torah reading for Purim, and for the haftorah we repeat the story of Saul and Amalek—that was read last week on Parshat Zachor.

Sunday:

We send mishloach manot,6 and enjoy the Purim feast7 during the daylight hours of Sunday.

Solidarity with Jerusalem

“It would therefore be fitting and good, very good, that on this Sunday [when Jerusalemites celebrate the ‘third day of Purim’] Jews everywhere should add in joyful activities: words of Torah (which ‘gladden the heart’), gladdening other Jews with a feeling of love, and, if fitting or necessary, through sending mishloach manot and giving gifts to the poor . . .

“And through this all to further unite with Jerusalem, to which we turn daily during the course of every prayer: ‘And they will pray to G‑d by way of the city that You chose’8—chosen and given to each and every Jew forever, an eternal inheritance.”9