The fourteenth and fifteenth of Adar are celebrated as Purim. The specific day on which Purim is celebrated depends on the location; in places where Purim is celebrated on the fourteenth, it is not celebrated on the fifteenth, and vice versa.

The original observance of Purim as a festival was established by the Sages and the prophets...

The original observance of Purim as a festival was established by the Sages and the prophets, who decreed that it be observed in every generation, as we read in the Megillah [Scroll of Esther]: "To establish these days of Purim at their appropriate times" (9:31). The Talmud explains: "these days...at their appropriate times—i.e., the time appropriate for one [city] is not appropriate for the other [city]." (Megillah 2b)

Why were different days established as Purim in different cities? Why wasn’t one day chosen as Purim in all cities, just as other festivals are celebrated on the same day in every city?

We find that even in the times of Mordechai and Esther, Purim was celebrated on a different day in Shushan than in the other cities. In all other cities, the battle against the enemies of the Jews took place on the thirteenth of Adar, and the people rested and celebrated on the fourteenth of Adar. In Shushan, however, the battle took place on the thirteenth and fourteenth of Adar, and the people rested and celebrated only on the fifteenth [see Esther 9].

In principle, then, only the city of Shushan should celebrate on the fifteenth of Adar, for it was only there that Purim was celebrated on that day. The sages of that era, however, wished to accord honor to Jerusalem and to the Land of Israel which was desolate at the time.

They therefore issued the following ruling: Shushan, where the miracle occurred, has an importance of its own and celebrates Purim on the fifteenth, even though it was not settled and thus did not have a surrounding wall at the time when Yehoshua bin Nun (Joshua) conquered the Land of Israel.

Other cities which were settled and had walls at the time of Joshua...are considered to be important...

Other cities which were settled and had walls at the time of Joshua, even if they are in a state of ruin and are no longer surrounded by walls, are considered to be important, and their present state of destruction (when the Sages established the holiday of Purim) is considered to be temporary. They are therefore accorded the status of Shushan and celebrate Purim on the fifteenth. Cities which did not have surrounding walls at the time of Joshua, even if they have walls surrounding them today, are to celebrate Purim on the fourteenth of Adar.

All of the above applies to cities both in the Land of Israel as well as outside it.

Hence, the Purim which is celebrated on the fourteenth of Adar is referred to as Purim of the "open" [i.e., unwalled, or less important] cities, whereas the Purim which is celebrated on the fifteenth is referred to as Purim of the "walled," major, cities.

Today, the only city in which Purim is celebrated on the fifteenth of Adar (besides Shushan) is Jerusalem. Although the Megillah is also read on the fifteenth of Adar night and day in a number of other cities in the Land of Israel, including Safed*, this is only a custom based on the uncertainty whether they were surrounded by walls at the time of Joshua. In these places, the Megillah reading on the fifteenth is done without reciting the blessings. For all other purposes, these cities celebrate Purim on the fourteenth.

[Excerpted from The Book of Our Heritage (Feldheim)].

* Editor’s note:
The other cities are Hebron, Tiberias, Acre, Ashdod, Ashkelon, Beersheva, Beit She'an, Beit Shemesh, Gaza, Gush Halav, Haifa, Jaffa, Lod, Ramlah and Shechem. (Some say that Jewish communities in Baghdad, Damascus and Prague also used to hold an additional megillah reading on the 15th with no blessings.)