Rosh Hashanah, Chapter 2, Mishnah 9. In the previous Mishnah, Rabban Gamliel and Rabbi Yehoshua disagreed: witnesses came and testified that they had seen the new moon on the earliest possible day. Rabbi Yehoshua agreed with the view that these were false witnesses, and therefore that day was not Rosh Chodesh - which in that case was also Rosh Hashanah - but rather Rosh Chodesh and Rosh Hashanah fell on the second day. Rabban Gamliel, however, who was the Nasi of the Sanhedrin, held that the witnesses were truthful, and therefore Rosh Hashanah fell on the first day.
The Nasi's Decree:
Rabban Gamliel sent word to Rabbi Yehoshua: "Gozerani alecha shetavo etzli b'maklecha uv'ma'osecha b'Yom haKippurim shechal lihyos b'cheshbonecha" - I decree upon you that you come to me with your staff and your money on the day that Yom Kippur falls according to your own reckoning, which according to Rabban Gamliel's reckoning was the day after Yom Kippur.
The commentators explain the reason for this: Rabbi Yehoshua was second in importance in the Sanhedrin, and this dispute was liable to lead to a split in the Sanhedrin. Rabban Gamliel wished to prevent that in advance and to make certain it would not happen, and therefore he compelled Rabbi Yehoshua to come to him while desecrating the day that Rabbi Yehoshua held to be Yom Kippur. Rabbi Yehoshua felt bound to comply, and he was left with no choice.
Rabbi Akiva's Words:
The Mishnah relates: Rabbi Akiva went and found Rabbi Yehoshua distressed. He sought to encourage him and said to him: "Yesh li lilmod shekol mah she'asah Rabban Gamliel asuy" - I have a basis to teach that whatever Rabban Gamliel does is done, meaning that this is the law. If so, you have nothing to worry about: even though you disagree, it is not relevant. Once he ruled and established that the first day is Rosh Hashanah, that is what it is.
And what is the source? "Eleh mo'adei Hashem mikra'ei kodesh asher tikre'u osam" - these are the festivals of the Holy One, blessed is He, that are called holy, and you are the ones who proclaim them, whether at their proper time or not at their proper time. "Ein li mo'ados ela eilu" - the Holy One, blessed is He, says: I have no festivals other than those which you proclaim. Rabban Gamliel is the one in charge; he declared that this is Rosh Hashanah, and therefore this is Rosh Hashanah. It makes no difference that you disagree, and you need not be concerned that you are desecrating your Yom Kippur - it is not even your Yom Kippur, since you must follow the ruling of the Sanhedrin, even if it is mistaken.
Rabbi Dosa ben Harkinas's Words:
Rabbi Yehoshua came to Rabbi Dosa ben Harkinas, who was the one who had said in the first place that they were false witnesses. Even though he maintained his opinion that they were false witnesses, he disagreed with Rabbi Yehoshua's sadness and said to him: if we come to question the court of Rabban Gamliel and to say who knows whether he is right, we will have to do the same to every single court that has stood from the days of Moshe until now.
And from where do we know this? For it says regarding the giving of the Torah: "And Moshe and Aharon, Nadav and Avihu, and seventy of the elders of Israel went up." Why does the Torah not list the names of the elders, leaving them anonymous? To teach us that any three who have been appointed as a court over Israel have the same standing as the court of Moshe. We do not know who they were, and perhaps they were no more distinguished than the court of Rabban Gamliel. The Torah teaches us that we are not to evaluate the standing of the court or the question of whether they are right or mistaken; the law is that we follow that court regardless. For this reason there is no room to challenge Rabban Gamliel's authority.
The Conclusion of the Story:
Rabbi Yehoshua took his staff and his money in his hand and went to Yavneh, to Rabban Gamliel, on the day that Yom Kippur fell according to his own reckoning. And here one of the most beautiful scenes takes place: when he arrived, Rabban Gamliel stood up in his honor, kissed him on his head and said to him: "Bo b'shalom rabbi v'salmidi - rabbi b'chochmah, v'salmidi shekibalta es devarai" - come in peace, my teacher and my student: my teacher in wisdom, and my student in that you accepted my words and upheld what the court said.
In summary: In this Mishnah we learned about Rabban Gamliel's decree upon Rabbi Yehoshua to come to him with his staff and his money on the day Yom Kippur fell according to his own reckoning, out of a desire to prevent a split in the Sanhedrin. Rabbi Akiva taught him from the verse "asher tikre'u osam" that whatever Rabban Gamliel did is done, whether at the proper time or not at the proper time; and Rabbi Dosa ben Harkinas taught him that one does not challenge the authority of a court, for any three who have been appointed as a court over Israel are like the court of Moshe. And it was the acceptance of the ruling that brought about the mutual honor at the end: "Come in peace, my teacher and my student."