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A unique halakha pertaining to Purim is that it has two dates. To understand the reason for this distinction, we must first recall the order of events. Haman’s edict stated that all the antisemites would be permitted to kill the Jews on the thirteenth of Adar. With Mordecai and Esther's ascension to power, the edict was modified to permit the Jews to defend themselves and kill their enemies. They did so on the thirteenth of Adar. On the next day, the fourteenth of Adar, they celebrated. However, in Shushan the capital, there were so many antisemites the Jews were granted an extra day to fight back. Therefore, in Shushan, their victory celebration took place on the fifteenth of Adar.
To commemorate the two days of celebration, the Sages ruled that Purim should be celebrated on the fifteenth in all important cities surrounded by walls, like the capital city of Shushan, and on the fourteenth everywhere else. At the time of the Purim story, the Land of Israel was desolate and had no walled cities. Nevertheless, to honor the Land of Israel, the Sages decided that cities walled at the time of Joshua son of Nun would celebrate Purim on the fifteenth.
In the 2500 years that have passed since then, traditions have been lost about which places had walls at that time, and some cities’ status is doubtful. There is only one city for which we still have a clear tradition as to its walled status during the times of Joshua – Jerusalem, our holy and glorious city. Only there does everyone celebrate Purim on the fifteenth. Cities whose status is unclear (such as Hebron, Tiberias, Jaffa, and Lod) celebrate Purim on the fourteenth. Residents of those cities who wish to enhance the mitzva celebrate both days.[1]
[1] In some years, when the fifteenth of Adar is on Shabbat, Jerusalem experiences an unusual Purim. It is called Purim Meshulash (“Triple Purim”) because the mitzvot of Purim are divided over three days. On Friday, we read the Megilla and give matanot la-evyonim. On Shabbat, we recite Al Ha-nisim and the Torah reading includes the passage normally read on Purim. On Sunday, we eat the festive meal and give mishlo’aĥ manot.
