The last Mishna in our perek relates the famous story of Rabban Gamliel and Rabbi Yehoshua, who disagreed one year about when Rosh HaShana fell out. Witnesses came to testify, claiming that they had seen the new moon on the 30th day (which would have made Elul 29 days long), but that the moon did not appear on the next night. Rabban Gamliel accepted their testimony, but Rabbi Yehoshua agreed with Rabbi Dosa ben Horkinas that their testimony could not be accepted, as it had to be false.
Rabban Gamliel’s position is explained by the Rambam as being based on the fact that the first testimony of the witnesses was acceptable, and that the testimony about the next day was irrelevant since there probably was a cloud, fog or some other factor that kept the witnesses from seeing the moon. Rav Zerahya HaLevi suggests that Rabban Gamliel was mistaken in his ruling, and that the witnesses, in fact, were unreliable. Nevertheless, Rabban Gamliel felt that for a variety of reasons he could not rescind his original ruling.
In any case, when Rabban Gamliel heard that Rabbi Yehoshua disagreed with him, he ordered him to appear carrying his walking stick and satchel on the day that Yom Kippur would fall according to his (Rabbi Yehoshua’s) figuring. Rabbi Akiva, who saw the distress suffered by Rabbi Yehoshua, consoled him by offering a teaching in support of the finality of Rabban Gamliel’s ruling. The passage (Vayikra 23:4) is understood to mean that the holidays are established based on the decisions of the court, even if that decision is incorrect because of error, because the court was misled, or even if they purposefully made the wrong ruling.
The baraita records that Rabbi Yehoshua thanks Rabbi Akiva for his insight, saying “you have consoled me, you have consoled me.” This double expression of consolation is understood to refer both to Rabban Gamliel’s error and to the fact that Rabbi Yehoshua will be obligated to violate Yom Kippur, according to his own beliefs.
Our perek concludes with the story of Rabbi Yehoshua appearing before Rabban Gamliel on the day that he (Rabbi Yehoshua) believed was Yom Kippur, carrying his walking stick and satchel as requested. Rabban Gamliel kissed him, calling him his teacher, for the Torah that he had learned from him, and his student, for having accepted his ruling.