All Jewish holidays are universally celebrated on the same day. To be sure, in chutz la'aretz, the Biblically-mandated holidays are celebrated for two days, but all start on the same day. Furthermore, the whole purpose of keeping two days there is because of sefeika d'yoma - a doubt which used to exist in chutz la'aretz as to which day is the correct day based on the inability of the agents informing the residents of chutz la'aretz of the declared date of the previous Rosh Chodesh to get everywhere on time - but the goal is to keep the correct day everywhere. That is, all Jewish holidays except for Purim where different days are mandated for different types of cities. As the Talmud (Megila 2b) based on a careful reading of Megilas Esther indicates, unwalled cities celebrate Purim on the fourteenth of Adar, whereas walled cities celebrate on the fifteenth. Why the difference between Purim and all other holidays? The Talmud (ibid. 5b) additionally teaches that elements of festivity apply on both days known as yomei d'Puraya, the days of Purim, regardless of which day is kept as the primary holiday. [1] Why are there two days of Purim for all?
Ramban[2] (Chidushim, Megila 2a) presents a novel interpretation in answer to this question based on a detailed reading of Megilas Esther providing additional insights into the nature of the miraculous salvation of Purim and the need for a dual holiday. The return to Eretz Yisrael had already begun through Zerubavel and was later rejuvenated by Ezra and Nechemia before the Purim salvation (see book of Ezra-Nechamia). Ramban opines that the majority of the Jewish people already resided there at the time of the Purim miracle mostly or wholly in unwalled cities, the walls having been destroyed during the Babylonian conquest with the exception perhaps of Jerusalem. (See Nechemia Chapters 3-4; Ramban though seems to assume that Jerusalem at that time was also unwalled or perhaps not fully walled.) The Jews throughout the lands of their exile lived both in unwalled cities (arei haperazim) and walled cities (mukafos choma): some of them exclusively Jewish; some of them mixed cities.
For apparent reasons, the residents of the unwalled cities felt the most vulnerable to Haman's decree, those in the walled cities less so as the wall would block the invaders from entering and allow battle against them from the wall. When the Purim miracle initially occurred, the Jewish residents of all cities outside Shushan celebrated on the day of their rest from battle, the 14th of Adar; the Jews of Shushan celebrated on the their day of rest, the 15th. However, in subsequent years only the Jews of the unwalled cities (and presumably the Jewish residents of the mixed, walled cities who were also under threat from their non-Jewish co-residents) spontaneously continued to celebrate on the anniversary of their rest day after their deliverance, on the 14th of Adar. However, the residents of the walled cities, not feeling as vulnerable and not viewing the miracle as great, did not celebrate at all in subsequent years.
Mordechai and Esther, finding an allusion in the Torah to ordain Purim as a permanent holiday (see Megila 7a and 14a) promulgated a two-fold Rabbinic ordinance: they codified the practice already begun by the Jews of the unwalled cities to celebrate on the 14th (See Esther 9:19,23), and they further enacted a new holiday on the 15th for all walled cities following the model of the initial day of rest of Shushan, which was a walled city as well. They did so since they realized that the decree was indeed looming over the entire Jewish people and a wall alone did not provide guaranteed shelter without Divine intervention. They divided the holiday into different days of observance to commemorate the greater miracle of the salvation of the Jews of the unwalled cities and their initiative to establish a holiday on the anniversary of that date but insisted that all Jews celebrate since they were all in danger (see Esther 9:20,24,30). Since the miracle primarily affected the land of Israel where most Jews lived and those cities were all (or mostly) unwalled and would not in any way partake of the distinctive holiday for the more prominent walled cities, the definition of a walled city was extended to include any city which was walled as of the time of Yehoshua bin Nun even if the wall was currently in ruins. Even the walled cities with mixed populations who were under obvious grave threat initially from their non-Jewish co-residents were included in the 15th day observance so as not to distinguish between the fully Jewish "walled cities" of Israel (which had a wall during Yehoshua's time), the main center of the miracle, and other walled cities.
Ran (on Rif 1a ff.) challenges Ramban's thesis on several points including debating historically whether the majority of the Jewish people already resided in Eretz Yisrael then and prefers a different, simpler approach. Since the initial salvation was on the 14th for most cities and continued until the 15th for Shushan, the Sages of that generation wanted to commemorate the additional miracle for Shushan by having all walled cities - similar to Shushan - celebrate on Shushan's holiday. The definition of a walled city was backed up to Yehoshua's time in order to honor the land of Israel, not because that is where most of the Jewish population lived, but to have some cities of Eretz Yisrael, the central land of the Jewish people regardless of how many live there, also have the distinction of "walled" cities.[3]
Rav Yitzchak Hutner (Pachad Yitzchak, Purim 15) explains the dual holiday on a hashkafic plane. As the Talmud (see Megila 7a) and many of the commentaries note, the miraculous salvation of the Jewish people also represented another step in the national battle against Amaleik, Haman and his cohorts being descendants of the first nation to battle against the Jews out of mere hatred of the sanctified life for which they stood. Some note that is for this reason that the Jewish victors did not partake of the booty (Esther 9:10) viewing it as assets of Amaleik from which it would be prohibited to benefit. However, the nature of the battle which took place on the 13th day of Adar, with the rest-day on the 14th, and the subsequent battle in Shushan on the 14th, with the rest-day on the 15th were fundamentally different. The initial battle was defensive, the Jews protecting themselves from the threat of Haman's willing agents; the second battle was offensive, after Queen Esther requested of Achashveirosh another day on which to take vengeance against the Jewish people's enemies. Am Yisrael's battle against Amaleik and the evil that this nation represents[4] takes on two forms. Sometimes Amaleik attacks first - as happened in the days of Moshe and Yehoshua - and Bnei Yisrael must then rally to defend themselves nationally or individually both on the physical and the spiritual plane. At other times, the Jewish people must pre-emptively strike Amaleik as happened in the days of King Shaul. These two crucial models for the eternal battle against this evil nation and what it represents are both incorporated into the dual holiday of Purim.
Rav Shmuel Bornstein (Sheim MiShmuel, Shemos 1, Purim p. 169 ff.) presents a second hashkafic explanation also connecting the dual celebration with the battle against Amaleik. The Talmud (Sanhedrin 20b) teaches that the battle against Amaleik is only commanded after a Jewish king is appointed. This is why Shmuel states "Hashem has commanded me to anoint you as king over Israel ... now, go out and smite Ameleik!" (Shmuel I 15:1,3). Sheim MiShmuel quotes his father, Rav Avraham of Sochatchov, who commented cryptically that the main miracle of Purim was that the Jewish people were able to wage war against Amaleik without a king. Explaining his father's words, Rav Shmuel elucidates that there must be Jewish unity for this battle to succeed since Amaleik has the power to divide the Jewish people; a king is necessary in order to accomplish this unity. How then did the Jewish people war against Amaleik in the days of Mordechai and Esther absent a Jewish king? The requisite unity was created via other means: the enormous threat of destruction looming over the heads of the Jewish people caused of all them to return together and nullify themselves to their Father in Heaven. This transformation from a disparate, scattered people referred to by Haman as "a nation scattered and separate among the nations" (Esther 3:8) was catalyzed by Esther's charge to Mordechai for three days of national fasting, prayer and repentance - memorialized by our observance of Ta'anis Esther - "go gather all of the Jewish people!" (ibid. 4:16). Only then, unified once again as they were formerly under a Jewish king, were the Jewish people able to wage the battle against Amaleik. Although outwardly the Jewish people were separated geographically and perhaps culturally throughout the many countries of their exile, inwardly they were indeed one.[5] Similarly, the two days of Purim, although outwardly they appear as different days of celebration, inwardly are really one, and hence, aspects of celebration apply to all on both days.
Many important lessons emerge from these various interpretations. Ramban enlightens us that one should never think that without Divine mercy and protection they are safe from external physical or spiritual enemies. Ran teaches us to appreciate each aspect of Divine miracles. Rav Hutner's teaching informs us, in addition to the literal defensive-offensive duality against the nation of Amaleik, to constantly stand on the defensive against spiritual laxity and lapses and be ready to go on the offense to constantly grow in our Avodas Hashem. Finally, Rav Bornstein illuminates the central concept of Jewish unity irrespective of external differences. May we be able to accept the Torah anew as the Jewish people did in the days of Mordechai and Esther (ibid. Shabbos 88a) this Purim and apply all of these concepts in our daily lives.
[1] Eulogies and fasting are prohibited on both (Megila 5b). Rema (Orach Chaim 695:2) additionally rules that one should "increase feasting and joy a little" on the other day as well.
[2] Ramban's opinion needs significant analysis as to his assumptions and the logical flow of his presentation. I present it here according to my humble understanding which to some extent is not in accordance with the way Ran apparently understood it.
[3] A Rav noted that this distinction given to Eretz Yisrael is now exclusively reserved for the Holy land as no walled cities from the days of Yehoshua have been positively identified in chutz la'aretz, and no Jews live currently in Shushan.
[4] See Amaleik and Purim: Deception and Self-Deception for one presentation of these concepts. Many more have been explicated by the commentaries.
[5] I once heard from Rav Herschel Welcher shlita that this is one of the reasons of the unusual mitzvah to drink on Purim. Wine has the quality of releasing inhibitions and fostering friendship even when ill-will existed between people previously.