Halakhah for Megillah 1:1
מְגִלָּה נִקְרֵאת בְּאַחַד עָשָׂר, בִּשְׁנֵים עָשָׂר, בִּשְׁלֹשָׁה עָשָׂר, בְּאַרְבָּעָה עָשָׂר, בַּחֲמִשָּׁה עָשָׂר, לֹא פָחוֹת וְלֹא יוֹתֵר. כְּרַכִּין הַמֻּקָּפִין חוֹמָה מִימוֹת יְהוֹשֻׁעַ בִּן נוּן, קוֹרִין בַּחֲמִשָּׁה עָשָׂר. כְּפָרִים וַעֲיָרוֹת גְּדוֹלוֹת, קוֹרִין בְּאַרְבָּעָה עָשָׂר, אֶלָּא שֶׁהַכְּפָרִים מַקְדִּימִין לְיוֹם הַכְּנִיסָה:
The Megillah is read on the eleventh (of Adar), the twelfth, the thirteenth, the fourteenth, and the fifteenth, [sometimes on one; sometimes, on the other, as explained below] — not earlier (than the eleventh) and not later (than the fifteenth). Cities surrounded by a wall from the days of Joshua the son of Nun read on the fifteenth, [it being written (Esther 9:19): "Therefore, the Jews of the outlying towns, who live in the unwalled cities, celebrate the fourteenth, etc." The unwalled cities, celebrating the fourteenth, the implication is that the walled cities celebrate the fifteenth. And "from the days of Joshua" is derived by identity: "perazi" ("unwalled," here) - "perazi" (Deuteronomy 3:5): "aside from the unwalled cities." Just as there, (perazi) from the days of Joshua the son of Nun; here, too, from the days of Joshua the son of Nun. And they ordained that the cities surrounded by a wall from the days of Joshua, even if they are not surrounded by a wall today, read on the fifteenth, like Shushan, in order to accord honor to Eretz Yisrael, which was in ruins in the days of Mordecai and Esther, that they, too, read as the men of Shushan and be regarded as if they were walled cities, even though now they are in ruins, so that there be a remembrance of Eretz Yisrael in this miracle. And Joshua is mentioned because he was the first who began to war against Amalek, viz. (Exodus 17:14): "Write this (the erasing of Amalek) as a remembrance in a scroll, and place it in the ears of Joshua, etc."] The villages and the large cities read on the fourteenth; but the villages may advance it (the Megillah reading) to the "day of assembly" (yom haknissah). [That is, since the walled cities read on the fifteenth, and the unwalled, on the fourteenth, all are included. How, then, could the eleventh, the twelfth, and the thirteenth obtain? The answer: The villages were permitted to advance their reading to the "day of assembly" — Monday or Thursday before the fourteenth — these (Mondays and Thursdays) being the days of assembly, when the villages assemble in the cities for judgment. For beth-din sit on Mondays and Thursdays by the ordinance of Ezra. Or it may be because the villages assemble in the cities on Mondays and Thursdays to hear the reading of the Torah. For the villagers are not so expert in the reading and need one of the men of the city to read for them; and the sages did not make them exert themselves to come back on the fourteenth, so that they be free on Purim to supply the needs of the Purim feast for the men of the cities. And they found an allusion for this in the Megillah, viz. (Esther (9:31): "to fulfill these days of Purim in their times" (bizmaneihem). If Mordecai and Esther instituted only the fourteenth and the fifteenth mentioned therein, we should have "zmanam" (connoting two times). Why "zmaneihem"? (connoting four times)? We are hereby apprised that two more times were added, aside from those mentioned in the Megillah. And it was not necessary for Scripture to include the thirteenth as fit for reading, because the miracle, essentially, occurred on that day. For it was on that day that the Jews gathered together to avenge themselves of their enemies, both in Shushan and in the other provinces. Perforce, then, Scripture adds only the eleventh and the twelfth. And it is not to be suggested that the sixteenth and seventeenth after the fourteenth and fifteenth written in the Megillah are intended, it being written (Ibid. 27): "and (the fifteenth) not to be passed by."]