הלכות מגילה א,י-יד

(10) [In a case of] a resident of a city that went to a [walled city]; or a resident of a [walled city] that went to a city: If he had in mind to return to his place at the time of the reading, but he delayed and did not return [in time] — he must read [on the date] like his place. But if he did not have in mind to return until after the time of the reading — he must read with the people of that place where he is [now]. And a [walled city] and all that is adjacent to it and all that can be seen with it — if there is no more than two thousand ells between them — is surely like the [walled city] and reads on the Fifteenth.

(11) A city which is a doubt — and it is not known whether it was surrounded by a wall at the time of Joshua bin Nun or it was surrounded afterwards — reads on both days, which are the Fourteenth and Fifteenth and on their nights. And they recite the blessing on its reading only on the Fourteenth, since that is the time of its reading for most of the world.

(12) [If] they read the Scroll during First Adar; and subsequently the year was made a leap year by the court — they must read it again during Second Adar at its time.

(13) The Megillah must not be read on the Sabbath. This is a precautionary measure, lest someone might take the Megillah to a skilled reader, transporting it four cubits or more through a public domain; for, though everyone is required to read the Megillah, not everyone is a skilled reader. For this reason, if the date for the reading of the Megillah falls on the Sabbath, it should be read prior to the Sabbath, while on the Sabbath itself the rules concerning Purim should be thoroughly discussed as a reminder that the day is Purim.

(14) If, for example, the fourteenth falls on a Sabbath, the inhabitants of open towns should read the Megillah on the preceding Friday, while the inhabitants of walled cities should read it on Sunday, their normal time. If the fifteen occurs on a Sabbath, the inhabitants of fortified cities should read it earlier, on Friday, which is the fourteenth, while the inhabitants of open towns should read it on the same day, which is the normal time for them. In such a case, all will be reading the Megillah on the fourteenth.