As we learned in a mishna: On that day, when they appointed Rabbi Elazar ben Azarya, Rabbi Akiva taught: “And every earthenware vessel into which any of them falls, whatever is in it shall be impure [yitma], and you shall break it” (Vayikra 11:33). The verse does not say: It is impure [tameh]; rather, it says: It shall be impure [yitma], indicating that an item in an impure earthenware vessel transmits impurity to other items. This verse teaches about a loaf with second-degree ritual impurity status, i.e., ritual impurity imparted through contact with a vessel impurified by a creeping animal, that the loaf renders other items impure with third-degree ritual impurity, even non-sacred items.
The Mishna says that this law was taught bo ba-yom – on “that day.” It appears that the expression bo ba-yom throughout the Mishna refers to the day that the sages decided to remove Rabban Gamliel from his position as Nasi and replaced him with Rabbi Elazar ben Azarya. On that occasion quite a few changes took place in the beit midrash and its organization. The Gemara in Berakhot (28a) records that new benches needed to be added to the study hall – some say 400; some say 700 – because Rabbi Elazar’s “open admissions” policy brought many new people who had been turned away under Rabban Gamliel. The Gemara reports that every issue in halakha that had been left in doubt was debated and resolved on that day, leaving a collection of statements throughout the Talmud that were recorded bo ba-yom.
According to the Gemara in Berakhot, although Rabban Gamliel lost his position because of the sages’ reaction to his treatment of his colleague Rabbi Yehoshua, he did not absent himself from the discussion while the debates took place, and the Gemara even records that he argued as an equal with Rabbi Yehoshua on that occasion. His conclusion from the episode was to recognize that he owed Rabbi Yehoshua an apology. In the end, the sages decided to divide the position of Nasi between him and Rabbi Elazar ben Azarya.
It appears that the change in leadership also led to greater openness in discussions, and opinions that had not been considered before were presented for debate. Similarly, new interpretations of Biblical passages were suggested and discussed. This is clearly shown in Massekhet Eduyot, which is made up of a series of Mishnayot that are a collection of testimonies about halakhic positions that had not been studied before, all of which were taught bo ba-yom – on “that day.”