In Jerusalem and other ancient cities, which were surrounded by a wall in the days of Joshua son of Nun, Purim is celebrated on the fifteenth of Adar, the day after it is celebrated in all other places. The reason for this is explained in the Book of Esther itself (9:15-19): The Jews fought their enemies on the thirteenth of Adar and rested from the war on the fourteenth, and therefore the fourteenth was established as a day of celebration and joy. By contrast, in Shushan the capital, the Jews were given permission to avenge themselves on their enemies on the fourteenth of Adar as well, and they rested from battle only on the fifteenth of the month. Consequently, when the holiday was established, it was set for everyone on the day that they rested from the war. The inhabitants of all other cities celebrated Purim on the fourteenth of Adar, whereas the residents of Shushan celebrated it on the fifteenth, and that day was called Shushan Purim. As the city of Shushan was surrounded by a wall, the Sages ruled that all cities surrounded by a wall in the days of the biblical Joshua have the status of Shushan in this regard, and they too celebrate Purim on the fifteenth of Adar (which is also called “Purim of the walled cities”). Anyone who has doubts about the date on which he should celebrate Purim should ask his local rabbi.

In principle, there is no difference between the laws and customs of Purim in other places, celebrated on the fourteenth of Adar, and the laws and customs of Purim in Jerusalem and other walled cities, on the fifteenth.

When the fifteenth of Adar occurs on a Shabbat, the mitzvot of Purim of the walled cities are spread out over three days, as detailed below. This Purim is called Purim Meshulash, “tripartite Purim.” In such a case, one who is celebrating in Jerusalem or other walled cities proceeds as follows: The megilla is read as in all other places, on Thursday night and Friday morning. On Friday, one gives the gifts to the poor. The Al HaNisim prayer is inserted into the Amida and Grace after Meals only on Shabbat. In the morning service of Shabbat, two Torah scrolls are taken out of the ark; seven men are called up for the weekly Torah reading from the first scroll, while an eighth person is called up for the Purim reading (Exodus 17:8-16) that is read for maftir from the second scroll. This is followed by the haftara (I Samuel 15:2–34), which discusses the battle of Saul against Amalek. This is the same haftara that was read for Shabbat Zakhor. This Shabbat has a certain atmosphere of joy, and conversation centers around the megilla. On Sunday, one fulfills the mitzvot of sending portions of food and “feasting and joy” with a large meal, including the drinking of wine. Thus, all the mitzvot of Purim are performed over the course of three days.

Further reading: For more on the month of Nisan, see A Concise Guide to Mahshava, p. 48.