We’ve learned that while Purim falls each year on the 14th of Adar, the megillah can be read as early as the 11th of Adar and as late as the 15th. Why the range? As we saw two days ago, it’s because small towns and villages move the reading of the megillah to the previous market day (Monday or Thursday) and walled cities celebrate Purim on the 15th.
Today’s daf inquires about what happens if Purim falls on Shabbat:
If the fourteenth occurs on Shabbat, the villages advance their reading of the megillah to the day of assembly (Thursday) the large towns read it on Shabbat eve (Friday), and the walled cities read it the next day (Sunday).
Rabbi Yehuda HaNasi disagrees with regard to large towns:
Since large towns were already deferred from their usual date (14 Adar), they are deferred to the day of assembly (the previous Thursday instead of Friday).
The discussion that follows is a complicated one, but the larger point is easy to grasp — under certain circumstances, we shift the observance of a holiday to accommodate the calendar, social conventions, or some other need. For those in the United States, this practice is a familiar one. When the Fourth of July falls on a Sunday, the federal holiday shifts to Monday, July 5th. In such a situation, towns that ordinarily host fireworks the night of the 3rd (so kids can stay up late the evening before a holiday instead of a school night) may choose to move their celebration to the evening of the 4th.
As the Gemara sorts out the differing opinions and the sources that support them, it notes that:
Everyone agrees that one does not read the megillah on Shabbat.
Rabba said: Everyone is obligated to participate in reading the megillah on Purim and not everyone is proficient in reading the megillah. Therefore, the sages issued a rabbinic decree that the megillah is not read on Shabbat, lest one take the megillah in his hand and go to an expert to learn how to read it or to hear the expert read it, and, due to their preoccupation, they will carry it four cubits in the public domain, and thereby desecrate Shabbat.
The decision not to celebrate Purim on Shabbat has nothing to do with the celebration of Purim itself. Rather, it was made to prevent us from inadvertently violating Shabbat — in this case, by inadvertently carrying a megillah in public. The rabbis use a similar argument to defer shofar blowing on Rosh Hashanah and lulav waving on Sukkot when these holidays coincide with Shabbat.
While we move the observance of Purim when it falls on Shabbat, it does not mean that we ignore the fact that it’s Purim. Rather, we learn about the holiday and study texts that relate to it. As Rabbi Yehoshua ben Levi teaches:
When Purim occurs on Shabbat, one asks questions and expounds upon the subject of the day.
It’s interesting to note that since the standardization of the Jewish calendar, the 14th of Adar never falls on Shabbat (or Monday or Wednesday for that matter). So, except in walled cities, like Jerusalem, this entire debate is moot. And if the 15th of Adar falls on Shabbat? Walled cities celebrate Purim on Friday the 14th like everybody else.
Read all of Megillah 4 on Sefaria.