39 Rosh Hashana 21a

The Gemara answers: It is a rare occurrence that two consecutive months are made short, and this would generate publicity, so that everyone would know about it. § It was related that Levi once arrived in Babylonia on what was observed there as the eleventh of Tishrei. He said: How tasty is the dish of the Babylonians on the great day of Yom Kippur, as they are observing Yom Kippur in the West, Eretz Yisrael. The month of Elul had been declared full in Eretz Yisrael, and according to the calendar there, it was only the tenth of Tishrei. They said to him: Testify that today is Yom Kippur and we shall observe it. He said to them: I myself did not hear the court proclaim: It is sanctified. Although I know that the month had been declared full, since I did not personally hear the proclamation, I cannot offer direct testimony such that you should change your calculations. It was further related that Rabbi Yoḥanan used to proclaim: Anywhere that can be reached by the messengers who go out in Nisan in time to inform the people when to observe Passover, but cannot be reached by the messengers sent out in Tishrei, let them also observe the festival of Passover for two days. The messengers did not travel on Rosh HaShana or Yom Kippur, and therefore they could travel three days further in Nisan than in Tishrei. The Sages instituted that two days must be observed in Nisan as a rabbinic decree due to Tishrei, for if they observe Passover for only one day, they will come to observe Sukkot for one day as well, and this they are not permitted to do. It was reported that Rabbi Aivu bar Naggarei and Rabbi Ḥiyya bar Abba, two disciples of Rabbi Yoḥanan, once arrived in a certain place that could be reached by the messengers who go out in Nisan, but could not be reached by the messengers who go out in Tishrei. And they saw that the locals observed only one day of Passover. They said nothing to them to correct their practice. Rabbi Yoḥanan heard this and he became angry with Rabbi Aivu and Rabbi Ḥiyya, for they had failed to rebuke the people who were acting contrary to Rabbi Yoḥanan’s explicit ruling. He said to them: Did I not say to you that anywhere that can be reached by the messengers sent out in Nisan but not by those sent out in Tishrei must observe two days of Passover, for the Sages instituted a rabbinic decree in Nisan due to Tishrei? § The Gemara relates that Rava would regularly sit in observance of the fast of Yom Kippur for two days, in case Elul had been declared a thirty-day month and Yom Kippur should be observed on what was observed in Babylonia as the eleventh of Tishrei. It once happened in accordance with his opinion. Elul had been declared a thirty-day month, and he was the only one who observed Yom Kippur on the correct day. It was related that Rav Naḥman had once fasted the entire day of Yom Kippur as usual. In the evening, toward the end of his fast, a certain man came and said to him: Tomorrow is the great day, Yom Kippur, in the West, Eretz Yisrael, and it is therefore necessary to fast tomorrow. Rav Naḥman said to him: From where do you come? He said to him: From a place called Damihareya. He said to him, playing on the name of his place: Blood will be his end, meaning Rav Naḥman’s own end. Due to this information, Rav Naḥman would have to fast two successive days, and thereby suffer greatly, as if his blood were being shed. He read the verse about him: “Our pursuers were swifter than vultures in the sky” (Lamentations 4:19), for had this messenger arrived just a little bit later, they would have eaten and drunk in the meantime. § Rav Huna bar Avin sent this instruction to Rava: When you see that, according to your calculations, the season of Tevet, i.e., winter, will extend to the sixteenth of Nisan, and the spring equinox will occur after the sixteenth of Nisan, add an extra month to that year, making it a leap year. And do not worry about finding an additional reason to justify making it a leap year, as it is written: “Observe the month of spring” (Deuteronomy 16:1). That is to say, see to it that the spring of the season, i.e., the spring equinox, is in the new part of Nisan, i.e., the first half, before Passover. It was related that Rav Naḥman said to those setting out to sea before Nisan: Since you will not know the determination of the first day of the new month, this is what you should do: When you see that the moon sets at daybreak, i.e., that it is visible all night from sundown to sunrise, know that it is the middle of the month of Nisan and burn your leaven. The Gemara asks: When does the moon set at daybreak? On the fifteenth of the month. But on the fourteenth of Nisan we burn leaven. The Gemara answers: For those out at sea, to whom the world is revealed, to whom the horizon is wide open and clearly visible, the moon completes its course at sunrise already on the fourteenth of the month. They can therefore rely on this sign to establish the date of Passover and the time for burning leaven.