The day of Ereb Rosh Hashanah is a particularly auspicious time for Teshuba (repentance). The Midrash comments that one-third of our sins are atoned on Ereb Rosh Hashanah, another third is forgiven during the Aseret Yemeh Teshuba (Ten Days of Repentance), and the final third is atoned on the day of Yom Kippur. Thus, Ereb Rosh Hashanah has special significance as an occasion for atonement, and it should therefore be spent giving charity, studying Torah and reciting Tehillim.
Additionally, there is a custom observed by many to fast on the day of Ereb Rosh Hashanah. This practice is recorded already by the Tur (Rabbenu Yaakob Ben Asher, Germany-Spain, 1270-1340). One who wishes to observe this practice should declare his formal acceptance of his fast during Minha the previous afternoon. This means that just before reciting “Oseh Shalom” at the conclusion of the Amida, one declares that he accepts a fast from Alot Ha’shahar until nightfall the following day (“Hareni Be’ta’anit Mahar Me’alot Ha’shahar Ad Ha’layla”). If one did not make this declaration at Minha, his fast nevertheless counts as a formal Ta’anit, since it is a common custom and thus takes effect even without a formal declaration. Preferably, however, one should ensure to make this declaration at Minha the day before Ereb Rosh Hashanah.
If one feels unable to observe a complete fast, he may fast until Hasot (midday), and this, too, has value. In this case, as well, one should declare his acceptance of the fast at Minha the day before Ereb Rosh Hashanah (“Hareni Be’ta’anit Mahar Me’alot Ha’shahar Ad Hasot”).
If one feels unable to fast at all on Ereb Rosh Hashanah, he should at least involve himself in charity, prayer and Torah study, and perform Teshuba on this day, as discussed. The decision of whether or not to fast, or whether or not to fast the entire day, should of course be made on the basis of one’s physical condition and ability to properly function without eating or drinking.
The Zohar writes that on any fast day, one may not eat once he goes to sleep the night before. Even if he wakes up before daybreak, and even if he stipulated before going to sleep that he still wishes to eat, he may not, according to the Zohar, eat once he went to sleep. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) followed this ruling and felt very strongly that one should not eat anything on the morning of a fast day even if he wakes up early, before daybreak. Hacham Ovadia Yosef cites authorities who rule more leniently in this regard, and allow eating in the morning before daybreak, but he nevertheless concludes that it is proper to abide by the Zohar’s stringent position. Therefore, if a person took it upon himself to fast on Ereb Rosh Hashanah, he may not eat in the morning even though he arises early for Selihot. This Halacha applies only to eating; drinking is permissible before daybreak on a fast even if one had gone to sleep. Therefore, one may drink water or coffee, for example, if he wakes up for Selihot before daybreak early in the morning of Ereb Rosh Hashanah, even though he is fasting that day.
Summary: One should spend the day of Ereb Rosh Hashanah engaged in Torah, charity and prayer, as it is an auspicious time for repentance. Many people have the custom to fast on this day, and one who is able to observe this practice should do so, and should declare the formal acceptance of the fast during Minha the day before Ereb Rosh Hashanah. If one accepted the fast, he may not eat once he goes to sleep the night before Ereb Rosh Hashanah, even if he wakes up before daybreak. One may, however, drink until daybreak, even after he went to sleep.