The day of Ereb Rosh Hashanah is likely the most underestimated day on the Jewish calendar. The Midrash teaches that Ereb Rosh Hashanah has the capacity to atone for one-third of a person’s sins. The Ten Days of Repentance then atone for another third, and the final third is atoned for on Yom Kippur.
Due to the value and significance of Ereb Rosh Hashanah, there is a custom recorded in the Shulhan Aruch to fast on this day, even if it falls on Ereb Shabbat. This is certainly a proper custom to observe if one is healthy and capable of fasting without too much difficulty.
This fast does not require a formal declaration accepting a fast upon oneself beforehand. One who wishes to observe this custom simply refrains from eating starting from Alot Ha’shahar (daybreak) on the morning of Ereb Rosh Hashanah. He does not have to formally accept upon himself the fast during Minha the day before, as one does before voluntary fasts. However, if one does not formally accept the fast during Minha the day before, then he may not recite Anenu on the day of the fast.
Although the fast begins only at Alot Ha’shahar, the Zohar (principal text of Kabblalistic teaching) instructs that after one goes to sleep the night before a fast day, he may no longer eat, even if he wakes up before daybreak. The Zohar applies this ruling even if a person explicitly stated before going to sleep that he does not yet accept the fast and he plans to eat later in the night before daybreak. Drinking, however, is allowed before Alot Ha’shahar, even after one has gone to sleep. Therefore, if a person wakes up early for Selihot on Ereb Rosh Hashanah, before daybreak, and wishes to drink water, tea or coffee, he may do so. He should not, however, eat, even though he wakes up before daybreak.
When does the fast of Ereb Rosh Hashanah end?
This issue is subject to debate among the Halachic authorities. Some maintain that this fast is no different than any other fasts, and one may therefore not eat or drink until he recites Kiddush on the night of Rosh Hashanah. Others, however, claim that one may eat already at the point of Pelag Ha’minha late in the afternoon, and yet a third view allows one to break the fast even earlier. As for the final Halacha, one should fast the entire day, until Kiddush. However, when Rosh Hashanah falls on Shabbat, there is greater room for leniency if a person feels weak and does not wish to enter Shabbat in a state of discomfort. In consideration of the honor owed to Shabbat, it would be permissible for a person in such a situation to break his fast toward the end of the day, even before the onset of Shabbat and Rosh Hashanah. Otherwise, it is proper to fast the entire day, until nightfall.
Summary: There is a custom to fast on Ereb Rosh Hashanah, and those capable of fasting should observe this practice. The fast begins at daybreak on Ereb Rosh Hashanah morning, and continues until Kiddush that night. After one goes to sleep the night before Ereb Rosh Hashanah he may no longer eat, even if he wakes up before daybreak, but he may drink until daybreak. When Ereb Rosh Hashanah falls on Friday, it is permissible to break the fast late Friday afternoon, even before sundown, if he feels weak.