Rosh Hashanah 21 - Two days of Yom Kippur?
The Jews in Diaspora have a custom to observe Holidays for two days. This is done in commemoration of the fact that in the times of the Sanhedrin, when the Court determined the New Moon with the witnesses' testimony, the messengers of the Court might not bring the news on time, and the exact day of the Holiday in the Diaspora was uncertain.
By the same token, Yom Kippur should also be observed for two days, or forty-eight hours of uninterrupted fast. However, since the month preceding Yom Kippur (Elul) was usually short, it was quite certain when the real Yom Kippur was on the calendar. Nevertheless, Rava was used to observe Yom Kippur for two days in a row. It once happened that the court made Elul thirty days long, and Rava thus justified his suspicion. Rav Nachman was once ready to eat on the night following a complete Yom Kippur when a man said to him that the court has decreed Elul to be long, and therefore the next day was the true Yom Kippur. Rav Nachman, frustrated, applied to himself the phrase "our pursuers were quick," meaning that the man has come too fast, before Rav Nachman had a chance to eat.
Since the exact date of Passover was important to know so as not to eat leaven (something for which a person deserves being cut off from his people), the messengers of the court were allowed to violate Shabbat, but bring the news on time. The same also applied to Yom Kippur, because of the fast. Exactly what violations are allowed? Some say - all, others - only walking too far (beyond the 'techum' area of two thousand steps).
Art: Walking In The Woods by Emile Isenbart