inyanei halacha/lomdus for Purim

 1) Women are not obligated in the mitzvah of giving machtzis ha'shekel (see post here).  Therefore, b'pashtus it would seem that MG"A is correct in arguing that women have no obligation to give a zecher to the machatzis ha'shekel.  However, it seems that most contemporary poskim (see Chazon Ovadya on Hil Purim, R' Elyashiv and R' Shlomo Zalman and others quoted in the Dirshu M"B) hold otherwise.  I don't know when or why things shifted away from the MG"A's view (maybe because women are included in the protection of machatzis ha'shekel, as my wife explained, or maybe because as GR"A notes, giving on a taanis is a kiyum of tzedaka, not just a zecher to the shekalim), but kach hava.  

2) The Ran writes that if there is a safeik whether a city is mukaf or not, they should read megillah on the 14th based on rov.  The Shaar haMelech raises the question of why we don't say kol kavu'a k'mechtza al mechtza given that a city is stationary.  

The classic case of kol kavu'a (see end of first perek in Kesubos) is where you have 9 butcher shops selling kosher meat and one selling treif meat and you can't remember which store you bought your meat in.  Even though the odds are greatly in favor of the meat being kosher, there is a gezeiras ha'kasuv not to follow rov in this case since the stores are stationary.  If you found the piece of meat in the street, since the meat must have changed location from inside the store to outside, we say kol d'parish m'ruba parish.  

Some Achronim argue that the case of a city is different.  In the case of the meat, there are multiple butcher shops in the city you could have bought the eat from (kavu'a) or which the meat could have come from (parish).  There is a taaroves of items before us.  In the case of an isolated city, it is just one item standing on its own.  True, it is a stationary item, but that alone does not make it kavu'a.

There is an issur of planting in a nachal eisan where an eglah arufa was offered.  Since we don't know exactly where an eglah arufah might have been offered, why not prohibit planting every nachal eisan because kol kavu'a k'mechtza al mechtza?  Same type of question.  See Ohr Sameiach (ch 6 og Hil Avodah Zarah) 

3) The Megillah tells us (9:27) וְלֹ֣א יַעֲב֔וֹר לִהְי֣וֹת עֹשִׂ֗ים אֵ֣ת שְׁנֵ֤י הַיָּמִים֙ הָאֵ֔לֶּה כִּכְתָבָ֖ם וְכִזְמַנָּ֑ם.  The Rambam in Hil Mamrim (2:2) writes that if Beis Din makes a takanah, a later B"D which has more authority (greater in chochma and minyan) can overturn that gezeira.  Malbi"m explains our pasuk to mean that built into the takanah of Purim was the condition that it be an eternal celebration, not subject to review by any future B"D.

Lulei d'mistafina I would ask how the Malbi"m learned pshat in the first daf in Megillah where the gemara wants to know the source for reading on the additional days of 11, 12, 13 and not just 14 and 15.  Asks the gemara, דאי ס"ד אנשי כנה"ג י"ד וט"ו תקון אתו רבנן ועקרי תקנתא דתקינו אנשי כנה"ג והתנן אין ב"ד יכול לבטל דברי ב"ד חבירו אא"כ גדול ממנו בחכמה ובמנין ?  The implication is that lulei there was a future B"D that was indeed  גדול ממנו בחכמה ובמנין there would be no problem in changing the original takanah -- the takanah of Purim is no different than any other takanot Chazal.

Perhaps the condition was only not to eliminate the celebration on 14 and 15, but in other respects the takanah could be modified by a later, great B"D. 

4) Yesh lachkor whether there is one takanah to celebrate Purim, and whether you have to do so on the 14th or the 15th is just a detail in the kiyum mitzvah, or whether there were two separate takanaot -- a takanah to celebrate 14 Adar for those in unwalled cities; a second, separate takanah to celebrate 15 Adar in walled cities and Shushan.  

The Yerushalmi (2:3), on the one hand, writes that someone who is chayav to read on the 15th is yotzei b'dieved if he read on the 14th. Mashma that there is only one takanah, to celebrate Purim, and the rest is details.  Were reading on the 15th a separate takanah, it obviously can only be fulfilled on that day.   

On the other hand, the Yerushalmi also writes that if someone heard megillah on the 14th in a regular city but then travels to a walled city on the 15th, they have to hear megillah again.  If there is just one takanah to celebrate Purim and read megillah, why should the person have to celebrate 2x?  They already fulfilled the chiyuv on the 14th?  This Yerushalmi seems to suggest that celebrating on the 15th is an independent takanah, and therefore has nothing to do with what was done the day before.

How to get these two Yerushalmis to fit and whether or not the Bavli agrees with these dinim is a discussion in Achronim that you need a clearer head than I have on a fast day to digest : )

This chakira may explains the machlokes Rashi and Rosh with respect to someone who travels between cities.  According to Rashi, if you intend to be in an unwalled city on the morning of the 14th, you must read megillah on the 14th; if you intend to be in a walled city on the morning of the 15th, you must read with them on the 15th.  In theory, according to Rashi if one travels from an unwalled city on the 14th and spends the 15th in a walled city, one would have to read on both days.  Rosh disagrees and writes that the determining factor of when you have to read is what your location is on the morning of the 14th.  If you are in an unwalled city, you read then; if you are in a walled city, you read the next day on the 15th.  

According to Rashi, it would seem there are two separate takanot in play, and therefore one theoretically might have to read on the 14th and the 15th to fulfill each din.  According to the Rosh, there is one takanah to celebrate Purim, and it's just a matter of figuring out which day to do it on, either 14 or 15, but not both.

5) Baruch she'kivanti, I found the Raavyah here supports my suggestion that the Yerushalmi that speaks about bringing children to hear megillah is not a din in chinuch, but applies even to younger children because af hein ha'yu b'oso na'nes of being saved from destruction by Haman.  The GRA disagrees.  

It's not clear to me what the cutoff point or difference is between the age of "yode'i l'havin" (to quote Raavyah) where listening to megillah is meaningful as pirseumei nisa because of af hein as opposed to the age of understanding when the regular din of chinuch kicks in.