On the first page of Megilah, my chavruta and I were having concerns about the Mishna "No Beis Din may nullify the words of another Beis Din unless it is bigger in size and wisdom." While at first it did not seem like such a big deal to me, it does pose many problems and issues with later generations being able to set future law. And is, in reality, adding three days to the readings of Purim, a Rabbinic chag, nullifying their words? We nullify words of Torah (blowing Shofar on Shabbos) and yet there is no such similar concern - am I raising a good issue?
Herschel Hartz, Jerusalem, Israel
Dear Herschel
Thanks for your question. I would suggest that there is a major difference between the two cases. In the case of Shofar, Chacomim took a precautionary measure and prohibited blowing on Shabbos, lest someone come to carry the Shofar outdoors in violation of Shabbos. This is a Gzeirah which happens regularly. It is not establishing a new thing.
In the case of Megilah, the Gemorah explains that had the original mitzvah applied to 14th and 15th of Adar only, a later Beis Din could not establish that one is able to fulfill the requirement by reading on an earlier date. Such an act would be creating a new date where one can fulfill the mitzvah, something not established by the earlier Beis Din.
A freilichen Purim.
Y. Landy