Megillah, Chapter One, Mishnah One
The first mishnah of Megillah teaches that the Megillah might be read on different days, depending on the locality. Tomorrows mishnah will explain this in greater detail.
1) The Megillah is read on the eleventh, the twelfth, the thirteenth, the fourteenth, and the fifteenth [of Adar], never earlier and never later.
2) Cities which have been walled since the days of Joshua ben Nun read on the fifteenth; villages and large towns read on the fourteenth,
3) Except that villages move the reading up to the day of gathering.
Section one: This section provides all of the possible dates in Adar on which the Megillah might be read. Tomorrows mishnah will explain in what situation it might be read on the eleventh, twelfth, or thirteenth.
Section two: Esther 9:19 reads, That is why village Jews, who live in unwalled towns, observe the fourteenth day of the month of Adar and make it a day of merrymaking and feasting, and as a holiday and an occasion for sending gifts to one another. If Jews in unwalled towns celebrate Purim on the fourteenth, it implies that Jews in walled cities celebrate on another day. This day must be the fifteenth, since in verse 18 the Jews in Shushan rest from their fighting on the fifteenth.
The mishnah determines what is a walled city by reference to Joshua, even though Joshua lived hundreds of years before the events of Purim. The mishnah refers back to Joshua because the land of Israel was desolate in the time of Achashverosh and none of its cities were walled. In order to honor Israel, we therefore refer back to the original conquering.
Section three: Small villages move the reading up to the Monday or Thursday prior to the fourteenth of Adar. These were the market days, the days on which the court would convene and the days on which the Torah was read. The idea was that on these days the Jews would gather in the larger cities and it would be more possible to have a large celebration than if each individual village had celebrated separately on the fourteenth.