Yadayim, Chapter 3, Mishnah 5. Chapter 3 of our tractate has been occupied with the ways in which hands contract ritual impurity and with the items that convey such impurity to them. The chapter closes with a mishnah that carries something unusual: a date stamp. The ruling recorded here is not simply attributed to a sage, it is attributed to a particular day.
The voice in this mishnah is that of Rabbi Shimon ben Azzai, and he begins by marking his words as something handed down to him rather than reasoned out on his own: "Mekubal ani," I hold it as a received teaching, "mipi shivim ushnayim zakein," from the mouths of seventy two elders. He then anchors the tradition to a specific occasion, opening with "beyom shehoshivu," on the day when the sages placed Rabbi Elazar ben Azaryah "biyeshivah," at the head of the academy.
That gathering was far more than a routine day of learning. With Rabbi Elazar ben Azaryah newly installed at the head of the yeshivah, the assembled chachamim went through the halachic questions that had been left unresolved and settled them one after another. Matters that had long awaited a verdict received one on that occasion, and that is the reason so many rulings scattered through the Mishnah bear the stamp of this single date.
The Bartenura spells out the practical rule that emerges. Explaining the words "bo bayom," he identifies them with what happened to Rabbi Shimon ben Azzai, an event that occurred on the very day the sages placed Rabbi Elazar ben Azaryah at the head of the yeshivah. He then states a principle worth remembering: "Vechol makom shene'emar bamishnah," in every place where the Mishnah uses the phrase "bo bayom" with no further identification, "al oso yom ne'emar," it is speaking of that day, the day of Rabbi Elazar ben Azaryah's appointment.
So our mishnah does two things at once. It transmits a halachic tradition that ben Azzai received from the seventy two elders, and it places that tradition in the historical setting of the great day of decision in the academy. Once that setting has been introduced, the Mishnah is in a position to gather up other rulings that were issued on the very same occasion.