We have already learned that the Megilla is read on two separate days – on the 14th of Adar in a krah – an ordinary city – and on the 15th of Adar in an ir – a city that was surrounded by walls from the time of Yehoshua bin Nun. How do we establish whether a given individual is a ben krah or a ben ir? The Mishna teaches that someone who travels from one type of city to another becomes obligated according to his new city, assuming that he does not plan to return. If he plans to return, however, then he retains his original status and reads according to the tradition of the place that he is from.
Gemara: Rava said: They taught the Mishna that one who is destined to return to his own place reads according to the halakha governing his own place only with regard to one who is destined to return to his own place on the night of the fourteenth of Adar. But if he is not destined to return on the night of the fourteenth, although he does intend to return to his own place eventually, he reads with the residents of his current location.
Rava’s ruling is not as simple as it appears, and there are two main positions taken on how to understand it. Rashi understands it to refer to both a ben krah and a ben ir, and the question is whether the individual anticipates returning to his primary location by the time the Megilla will be read there. Thus, a ben ir who is visiting a krah on the 14th will not be obligated to hear the Megilla with the people of that town, assuming that he intends on returning home by the time that his ir will read the Megilla on the 15th. The Rosh disagrees, arguing that the crucial time is the 14th for everyone. We judge a person’s obligation by his position on the 14th when the basic obligation of keri’at Megilla begins. According to him, a ben ir who is visiting a krah on the 14th will be obligated to read on that day.
Another possibility that is raised by the poskim, based on the Talmud Yerushalmi, is that a person can find himself obligated on both days, or, perhaps, not obligated at all, if he travels between cities on the 14th and 15th.