The early sages (Tanaaim) drew distinctions between two types of cities on the matter of reading the Scroll of Esther: a walled city and an unwalled village. A city which was built and walled in the time of Joshua reads the megillah on the 15th of Adar. The scholars asked: Why did the early sages draw this distinction? The megillah itself draws a distinction between the city of Shushan and those who live outside of the city. Residents of Shushan were to celebrate Purim on the 15th of Adar and those who lived outside Shushan on the 14th. The Scriptures explain why. The residents of Shushan also killed gentiles on the 14th of the month, so they celebrated on the 15th, while Jews outside of Shushan killed gentiles only on the 13th, so they ecelbrated on the 14th. It is written “This was on the thirteenth day of the month of Adar. And on the fourteenth of the month they rested and made it a day of feasting and gladness. But the Jews who were at Shushan assembled together [and killed] on the thirteenth day, as well as on the fourteenth; and on the fifteenth of the month they rested, and made it a day of feasting and gladness” (Esther 9:17-18). Answer: One of the scholars, Rava, explained that it is written “Therefore the Jews of the villages who dwelt in the unwalled towns celebrated the fourteenth day of the month of Adar with gladness and feasting, as a holiday, and for sending presents to one another” (Esther 9:19). Since it says that Jews who live in unwalled villages celebrate on the 14th, the walled cities must celebrate on the 15th. The scholars asked: Perhaps the Scriptures meant, in specifying that the Jews of the villages celebrate on the 14th, that Jews in walled cities should not celebrate at all? Answer: This is not a reasonable interpretation, for no Jew would exempt himself from celebrating Purim. Also, the Scriptures explicitly state that all will celebrate, whether they live in a walled city or an unwalled city, for it is written “The remainder of the Jews in the king’s provinces [will celebrate Purim]” (Esther 9:16).
The king’s provinces stretched from India to Ethiopia and includes all the cities therein, be they walled or unwalled. The scholars went on to ask: It is possible that the Scriptures specifically state the unwalled villages celebrate Purim on the 14th to show that residents of walled cities should celebrate both the 14th and the 15th? Support for this supposition comes from the verse “To establish among them that they should celebrate yearly the fourteenth and fifteenth days of the month of Adar” (Esther 9:21) implying that there are cities, like those surrounded by walls, which must celebrate Purim twice annually. Answer: Since the word et separates the mention of the 14th from the mention of the 15th, this tells us we need celebrate Purim only for one day, each type of resident on the appropriate day. Residents of walled cities celebrate on the 15th of Adar and residents of unwalled villages on the 14th. The scholars then asked: Perhaps that verse specified only residents of unwalled villages to show that residents of walled cities are given the choice of when to celebrate Purim — they could choose to celebrate on the 14th or on the 15th? Answer: Since it is written in the megillah “To confirm these days of Purim at their appointed times” (Esther 9:31) with a plural noun [times] and not a singular [time], we see that each city has a specific date for Purim, and the date for walled cities is not the same as the date for unwalled villages. The scholars then asked: Perhaps the verse mentioned the 14th as the date for the unwalled villages to show that walled cities should celebrate Purim on the 13th of Adar? Answer: Since the residents of Shushan, capital of Persion, celebrate on the 15th of Adar, we see that residents of all other walled cities should also celebrate on the 15th of Adar.
(Babylonian Talmud, Tractate Megillah 2b)