Who would want observe Yom Kippur for two days? TB Rosh Hashana 21

A lunar month can either have 29 days (חסר) are 30 days (מלא). Before the calendar was set, witnesses had to go to the court in Jerusalem and testify that they had seen the new moon. After verifying their testimony, the court sent out messengers to inform people that are new month had arrived and be prepared to celebrate the holidays on the correct date. “They go out in the month of Nisan, due to Passover, so that people will know on which day to celebrate it; In six months of the year the messengers go out from the court in Jerusalem to report throughout Eretz Yisrael and the Diaspora which day was established as the New Moon, the thirtieth or the thirty-first day since the previous New Moon. They go out in the month of Nisan, due to Passover, so that people will know on which day to celebrate it; in the month of Av, due to the fast of the Ninth of Av; in Elul, due to Rosh HaShana, which begins thirty days after the New Moon of Elul; in Tishrei, due to the need to establish the correct dates on which to celebrate the Festivals of Tishrei, i.e., Yom Kippur and Sukkot; in Kislev, due to Hanukkah; and in Adar, due to Purim.in Elul, due to Rosh HaShana, which begins thirty days after the New Moon of Elul; in Tishrei, due to the need to establish the correct dates on which to celebrate the Festivals of Tishrei, i.e., Yom Kippur and Sukkot;” (TB Rosh Haashana 18a, Sefaria.org translation) The preceding month could either have had 29 days or 30 days and that was an important fact when comes to setting the calendar. The places where the messengers could reach in a timely fashion, celebrated the holidays for one day. The places where the messengers couldn’t reach celebrated the holidays for two days because of the doubt which day is actually the holiday.

The messengers could not travel on Shabbat or on the Festival to carry out their mission. This prohibition did not inhibit news to travel in time to celebrate Passover. However, this prohibition create problems in the month of Tishri. The amount of time allotted to travel was prescribed because the messengers were not permitted to travel on Rosh Hashanah and Shabbat. To avoid confusion and misunderstanding daf TB Rosh Hashana 21 teaches “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.” (Sefaria.org translation)

Celebrating Sukkot for two days is no great hardship, but what about Yom Kippur? Should we observe Yom Kippur for two days as well? There is a precedent on today’s daf. “§ 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.” (Sefaria.org translation)

I don’t have to tell anyone that we only observed one day of Yom Kippur because fasting two days in a row can be injurious to a person’s health. However, there is a disagreement concerning the custom to fast two days. There those who see that this is a custom of extraordinary piety (מנהג חסידות) while others prohibit it out right. In the Talmud Yerushalmi tells that Rabbi Yehoshua ben Levi saw fasting two days for Yom Kippur as something praiseworthy while Rav Hisda thought that one should not observe thusly. The Rama on his gloss reflecting Ashkenazic Jewry practice in the Shulkhan Aruk prohibits fasting two days of Yom Kippur. “יש מחמירים לעשות שני ימים יום כפורים ויש לזה התרה ואין לנהוג בחומרא זו משום דיש לחוש שיבא לידי סכנה [א"ז]” (Orekh Hayim, 624:5)