Yom Kippur is not observed for two days anywhere because of the difficulty of maintaining a fast over two days.In practice, the Sanhedrin had the discretion to arrange the month proclamations so that Elul would almost never be extended to 30 days. See BT Rosh Hashanah 19b, as well as commentators there. This greatly reduced the practical level of doubt as to which day would be the first day of Tishrei. The doubt still existed, so Rosh Hashanah and Sukkot were observed for two days. However, the low level of the doubt–combined with the difficulty of a 49-hour fast–led to the exemption of Yom Kippur from the requirement for a second day of observance. This complex issue is discussed more fully here.
Yom Kippur is not observed for two da.ys anywhere because of the difficulty of maintaining a fast over two daysIn practice, the Sanhedrin had the discretion to arrange the month proclamations so that Elul would almost never be .extended to 30 days. See BT Rosh Hashanah 19b, as well as commentators there. This greatly reduced the practical level of doubt as to which day would be the first day of Tishrei. The doubt still existed, so Rosh Hashanah and Sukkot were observed for two days However, the low level of the doubt–combined with the difficulty of a 49-hour fast–led to the exemption of Yom Kippur from the requirement for a second day of observance. This complex issue is discussed more fully here.