You may have noticed “Shushan Purim” on your Jewish calendar on the 15th of Adar. How is it different from regular Purim?
If you read the Book of Esther closely, you’ll see in Chapter 9:
However, the Jews of Shushan assembled on both the thirteenth and fourteenth days of Adar, so it was on the fifteenth that they rested and made it a holiday for celebrating and rejoicing. This is why the Jews of the villages, those who live in unwalled towns, make the fourteenth day of the month of Adar a day for celebrating and rejoicing, a holiday and a time for sending each other portions [of food].
So to this day, Jews in unwalled towns celebrate on the 14th of Adar, and Jews in walled cities celebrate on the 15th.
What cities are walled? Well, for these purposes only Jerusalem counts as a walled city, because a ruling in the Jerusalem Talmud, repeated in Maimonides‘ Mishneh Torah (Hilchot Megillah 1.5) says that we only count as walled cities those which were walled at the time that Joshua led us into the Land of Israel. This is done out of respect for the Land (which would have been in ruins during the period in which the Book of Esther is set). The mystic and sage Josef Caro added that this was also so that those of us in the Diaspora would never forget the Land of Israel.
So we wish a “Purim Sameach!” (“Happy Purim!”) to the Jerusalemites who are enjoying that holiday today. For the rest of us, Purim is done and now it’s time to get ready for Passover!