Rosh Hashanah 21A

תרי ירחי חסירי קלא אית להו.

The Talmud 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.

לוי אקלע לבבל בחדסר בתשרי. אמר: בסים תבשילא דבבלאי ביומא רבה דמערבא. אמרי ליה: אסהיד! אמר להו: לא שמעתי מפי בית דין ״מקודש״.

§ It was related that Levi once arrived in Babylonia on what was observed there as the 11th 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 should 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.

מכריז רבי יוחנן: כל היכא דמטו שלוחי ניסן, ולא מטו שלוחי תשרי ⁠— ליעבדו תרי יומי. גזירה ניסן אטו תשרי.

It was further related that R' 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 3 days further in Nisan than in Tishrei. The rabbis 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.