It is a Misva to involve in eating on Ereb Yom Kippur, and one who eats heartily on Ereb Yom Kippur is considered as having fasted both on Ereb Yom Kippur and on Yom Kippur. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) and the Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939), in Siman 604, write that one should take time from his Torah learning in order to eat festively on Ereb Yom Kippur.
This obligation applies equally to men and women.
Strictly speaking, one is not required to eat specifically bread on Ereb Yom Kippur, though it is widely accepted to follow the view of the Arizal (Rav Yishak Luria of Safed, 1534-1572) to eat bread, and to ensure to eat an amount that requires reciting Birkat Ha’mazon on the level of Torah obligation (a Ke’besa).
The Kaf Ha’haim records a custom to recite the Beracha of Ha’mosi over two loaves of bread on Ereb Yom Kippur, as we do on Shabbat.
According to the Kabbalists, one should eat on Ereb Yom Kippur the amount one would normally eat in two days. The Kaf Ha’haim adds that one should ensure to eat at least two meals with bread on Ereb Yom Kippur.
This Misva applies only during the day of Ereb Yom Kippur; there is no Misva to eat on the night of Ereb Yom Kippur.
Some have the custom to recite a "Le’shem Yihud" declaration before each meal on Ereb Yom Kippur.
According to some Halachic authorities, it is also a Misva to drink on Ereb Yom Kippur, as indicated in the Gemara’s formulation of this obligation ("Whoever eats and drinks on Ereb Yom Kippur…"). It should be noted that according to this view, one fulfills the Misva only by drinking significant beverages, such as wine, grape juice or beer, but not by drinking water, even flavored water. However, wine should be drunk only in small quantities, and only in the morning, so that one is not inebriated when Yom Kippur begins. This is the ruling of Hacham Bension Abba Shaul (Israel, 1923-1998).
One should eat a respectable, festive meal on Ereb Yom Kippur to celebrate our receiving forgiveness on Yom Kippur. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) wrote that it is proper to eat fish on Ereb Yom Kippur. (This is based upon a famous story told in the Midrash, which is mentioned by the Tur.) He adds that the fish should be eaten specifically in the morning of Ereb Yom Kippur.
Foods that increase the likelihood of nocturnal emissions should not be eaten at all on Ereb Yom Kippur. These include milk, garlic, eggs and sharp-tasting foods. The Mishna Berura notes that it is customary to allow drinking milk with coffee, as this is usually a small amount that will not have an effect upon a person.
Some authorities forbid taking pills before Yom Kippur to alleviate the discomfort of fasting, such as pills that release caffeine or certain nutrients and thus make fasting easier. These authorities claim that since the Torah requires "Ve’initem," that we "afflict" ourselves on Yom Kippur, it is forbidden to take pills for the sake of alleviating the discomfort of fasting. Halacha, however, does not follow this view and allows taking these pills before the fast, since the Halachic obligation is to abstain from eating and drinking on Yom Kippur, and not to cause oneself physical discomfort.