Megillah 4a-5a, from way way back on September 8:
We previously discussed the decree allowing rural people to read megillah earlier, moving the date up to Monday or Thursday, when they're in town anyway. But there's still no agreement on exactly why. Maybe it's to free them up on the day when the city slickers are observing Purim, so that they're available to supply food and water to the cities? But that's not it, because 14 Adar sometimes falls on Monday or Thursday anyway, in which case they'd be observing it on the same day. So the Talmud reluctantly concludes that maybe it's actually for the benefit of the farmers, rather than to make it easier to exploit the proletariat.
Normally, villages move the megillah reading up to the next Monday/Thursday before Purim, unwalled cities read on 14 Adar, and walled cities read on 15 Adar. But the megillah can't be read on Shabbat, under Rabbah's decree, intended to prevent anyone from carrying a megillah (or shofar or lulav) in the public domain. (And now I've learned it in all three masechtot! Score!) Therefore, when 14 Adar falls on Friday or Shabbat, we have some tough choices to make. Rabbah's decree isn't negotiable, but everything else is. The rabbis have to choose among several governing principles that coexist in harmony most years but become mutually exclusive when Shabbat comes into the picture:
- Unwalled cities can't move away from 14 Adar (unless they absolutely have to, due to Shabbat).
- Unwalled cities must read before walled cities.
- Walled and unwalled cities must not read on the same day.
- Walled cities must not read earlier than unwalled cities (note how this is different from the second one).
- John may not sit next to Mary.
If the megillah is read early, then matanot la'evyonim (gifts to the poor) are collected and distributed on that early date, since the poor people hear the megillah and get their hopes up. Simcha (festivity), on the other hand, only applies on the proper day. Don't let me catch you celebrating any earlier.
Rav Asi says that a minyan of 10 is always required to read megillah in order to properly publicize the miracle. Rav requires 10 if you're reading on an earlier date, but says that even a solitary individual can read megillah on the proper date. Rashi explains that megillah is an obligation on each individual on 14 Adar, so the individual is reading with a community even if s/he is all by him/herself, so the miracle is being satisfactorily publicized. Thus, those who are in remote locales and have difficulty finding a space-based community can still find a time-based community.
The next mishnah says that megillah is moved earlier (not later), but other things are moved later (not earlier), such as Tisha B'Av (who wants to move sad stuff any earlier?) and chagigah and hakheil (it doesn't count until the obligation has kicked in). (Then can't you say that the obligation of megillah hasn't kicked in yet either? That question isn't asked.)