Although Purim is generally celebrated on the 14th of Adar, it is celebrated on the 15th of Adar in cities that were walled during the time of Yehoshua, who led Beneh Yisrael’s initial conquest of Eretz Yisrael. Even though the city no longer has a wall, the date for its celebration of Purim is the 15th of Adar if it was walled during Yehoshua’s time. This applies both to cities in Eretz Yisrael and cities elsewhere. Since the Jews of Shushan, a walled city, celebrated their victory on the 15th, Jews in walled cities commemorate their celebration by observing Purim on the 15th of Adar instead of the 14th. Thus, in Jerusalem, Purim is celebrated 15th.
The question arises, however, as to why Hazal defined "walled cities" in this regard as cities that were walled in Yehoshua’s time. If the purpose of this Halacha is to commemorate the celebration in Shushan, it would seem that it should apply to cities that were walled in the times of Ahashverosh. What does Yehoshua have to do with the celebration of Purim?
The Talmud Yerushalmi explains that the Sages established this definition of "walled cities" in order to give respect to Eretz Yisrael. At time of Ahashverosh, there were no walled cities in Eretz Yisrael. Therefore, if Halacha would have singled out cities that were walled at the time of Ahashverosh, no cities in Eretz Yisrael would have received this special status, which would have been a slight to its honor. Therefore, Hazal established that all cities which had a wall in Yehoshua’s time are given this status and Purim is celebrated there on the 15th of Adar.
Hacham Ovadia Yosef, in Hazon Ovadia (p. 110), cites an additional reason from the Orhot Haim and Meiri. They explain that Yehoshua is associated with Purim because he was the first one to wage war against Amalek – the nation from which Haman descended. As we read in the Purim morning Torah reading, Yehoshua was appointed to lead the war against Amalek after Amalek attacked Beneh Yisrael. Therefore, in order to underscore this association, Hazal established that cities that were walled during his time would be singled out for the special celebration on the 15th of Adar.
