The Relationship between the Two Days of Purim
One of the more intriguing aspects of the mitzva of reading the Megilla is the variety of times during which this mitzva is performed. Generally, a mitzva is performed on the exact same date in every location. One exception, of course, is the second day of Yom Tov, which is kept in chutz la-aretz but not in Eretz Yisrael. However, in that instance the communities in chutz la-aretz keep an extended mitzva; all areas observe the same first day. Yet, in the case of Megilla, most people read on the 14th, residents of certain walled cities (mukafim) read on the 15th, while in the days of Chazal residents of small villages read as early as the 11th. How are we to view the relationship between the 14th of Purim and the 15th? Are they co-equal in their status as Purim? If they are, what is the relationship between them?
A famous piece by the Ramban in the beginning of Masekhet Megilla asserts a bold and novel approach to the history of Purim. He claims that during the period of the miracle, most Jews had already made aliya. Residing in Israel, those in walled cities had little fear of attack, for they could take refuge within the walls (something which would have been impossible in galut, since their enemies were living within those very same walls). Hence, the real danger was posed to the dwellers in non-walled cities (perazim) and, of course, in Shushan itself.
Initially, only the residents of non-walled cities commemorated Purim, since they had sensed the danger so deeply. After some time, Mordekhai and the Sanhedrin realized that Purim had Biblical support and should be universally instituted, and they therefore extended the celebration to all Jews. In recognition of the initiative taken by the non-walled residents, they retained their original day, while residents mukaf cities people who were 'latecomers' to the mitzva observance - were given a second day, Shushan's original day of the 15th.
This perspective helps explain a very perplexing statement of the gemara (2b). In attempting to derive a source for allowing reading on the 15th, the gemara cites a verse stating that perazi dwellers read on the 14th; if they read on the 14th, their counterparts in walled cities must read on the 15th. The gemara then counters: perhaps mukaf dwellers do not read at all! Why would the gemara consider the possibility of exempting walled cities from Megilla reading? The Rambans approach might provide an answer. Though the Ramban's perspective is historical in nature (he does not quote any distinct halakhot which are impacted by his view), it clearly establishes a hierarchy of days. The 14th is clearly the primary day of Purim, while the 15th was originally celebrated on the first year only by the people of Shushan, who were battling throughout the 14th. Ultimately after an interim of a few years - the 15th was selected as a secondary Purim to be celebrated by those who had not originally joined in the mitzva.
Similar sentiments lie at the heart of an interesting halakha cited by the Yerushalmi (Megilla 2:3) about travelers who, on Purim, are not located in either walled or open cities. Though one might imagine that the lack of alternate urban location would force them to read based upon the custom of their hometown, the Yerushalmi rules that they read on the 14th, which is zeman keri'ata the universal day of reading. It appears as if the Yerushalmi then quotes the dissenting opinion of Rav Mana that travelers would read based upon the customs of their home base, assuming they plan to return. (Astonishingly, this would imply that permanent nomads would not read the Megilla at all!)
This Yerushalmi is actually citing an earlier statement (1:1) that suggests an even more extreme position: if mukaf residents choose to read on the 14th, they have fulfilled the mitzva, since this is the primary day of reading. We certainly do not follow this position in Yerushalmi Megilla 1:1, and we certainly maintain that mukaf residents MUST read on the 15th. However, these two Yerushalmis, when taken together, yield the same result as the Ramban's historical analysis: the primary day of Purim is the 14th, with the 15th being defined as a secondary day.
The same Yerushalmi (2:3) that discussed the halakha for travelers raised another question which further highlights the relationship between the days. The Yerushalmi takes it as a given that a non-mukaf resident may not read the Megilla on behalf of mukaf residents on the 15th; since he is not obligated on that day, he may not read for those who are. May, however, a mukaf resident read on the 14th for those who live in open cities, since the 14th is clearly the primary day of Purim? Or do we say that since it is preferable for him to read on the 15th, he is not considered obligated on the 14th and may not read the Megilla on behalf of others? Clearly, once we assume the earlier Yerushalmi (1:1), the issue of whether a mukaf can read for a perazi on the 14th is rendered moot. If he may himself perform the mitzva on the 14th, he can certainly read for others.
It is difficult to gauge this question in the Bavli, which does not cite the various halakhot of the 14th mentioned in the Yerushalmi. The gemara (Megilla 2a) does state, "Zemano shel zeh lo ki-zemano shel zeh" (the time of the 14th is different from the 15th), but this is an ambiguous statement that does not necessarily reject the premise of the Yerushalmi. Tosafot (Yevamot 14a) cite the Yerushalmi about a perazi not reading for a mukaf on the 15th. However, they do not address the second part of the Yerushalmi, which discusses whether a mukaf can read for a perazi on the 14th. The Vilna Gaon (Orach Chayim 688) does indeed assert that a mukaf may read for a perazi on the 14th.
An interesting gemara in Megilla (19a), discussing those who change locations for Purim, presents a dispute that might touch upon our issue. The mishna mentions that people who visit on Purim, with intent to return home, should read as they would if at home. Rava comments that a person's location on the 14th is what really determines the timing of his reading. If a mukaf plans on returning home before the morning of the 14th, he should read on the 15th just as he would at home. If he plans on remaining past the morning of the 14th, he is temporarily defined as a perazi and reads on the 14th. The gemara does not discuss a situation of a perazi who travels to a walled city. Which moment is crucial in determining his status and his reading?
Rashi and the Ramban claim that for him, the morning of the 15th determines his status. If a perazi finds himself in a walled city on the morning of the 15th, he must read with them. Namely, day 14 is crucial in determining a status in an open city, while day 15 is equally important in determining status in a walled city.
The Rosh and the Ra'avad disagree, arguing that location on the morning of the 14th is the only decisive factor. If a mukaf remains in an open city on the 14th, he must read along with the locals on the 14th. Similarly, if a perazi is found in a walled city on the 14th, he is defined as temporarily mukaf and delays his reading until the 15th. Thus, according to the Rosh, the 14th alone is responsible for dictating the schedule of keri'at Megilla, whereas according to Rashi and the Ramban, each day is equally significant in establishing personal identity.
This dispute reflects the aforementioned question of whether the 14th and 15th are co-equal days, or whether the 14th is primary and the 15th ancillary. Interestingly, from a historical standpoint, the Ramban views the 14th as primary and the 15th as secondary; from a halakhic standpoint, however, they are indistinguishable - each can respectively establish temporary identity for travelers, forcing them to conform to the local customs of keri'at Megilla.