There is a mitzva to eat heartily on Erev Yom Kippur (SA 604:1), and according to most poskim, the mitzva is of Torah origin (MA and MB 604:1). The Sages said, “One who eats and drinks on the ninth [of Tishrei] – the Torah considers it as though he fasted on the ninth and tenth” (Yoma 81b). Even though fasting is harder than eating, God wants to increase our reward, so He considers our eating on the ninth as if it were fasting.
The reason for this mitzva is to prepare for the fast (Rashi) and to have the strength to pray properly on Yom Kippur (Sha’arei Teshuva 4:10). Therefore, one should eat heartily especially at the se’uda ha-mafseket, the last meal before the fast. This mitzva also expresses God’s love for us. He commands us to fast only one day a year to atone for our sins, but out of His abundant concern for our well-being, He commands us to eat and drink before the fast so that we will have the strength to get through it unharmed (Rosh; Tur OḤ 604). According to another view, even though eating before the fast is helpful physiologically, it has an element of inui (affliction), as it is difficult to transition from the pleasure of eating to the challenge of fasting (Shibolei Ha-leket; AHS 604:4).
Additionally, excessive eating is indeed an inui from the perspective of human spirituality, as it places an emphasis on the body. This, then, is precisely the point of the mitzva – to purify and improve the body as well as the soul, for perfect service of God is with the body and soul together. On the one hand, we must be wary of the evil inclination, which draws a person toward physical desires. On the other hand, we should not conclude that holiness is revealed only when the soul is alienated from the body and the physical world. It is specifically by highlighting the sacred value of physical enjoyment on Erev Yom Kippur that we can wholly repent on Yom Kippur (Shlah, Masekhet Yoma, Torah Or §136).
There is another important reason for eating on the ninth. All the Torah’s commandments must be undertaken with wholehearted joy, which includes physical enjoyment. Certainly, the mitzva of repentance should be done joyfully, since through it we are purged of all our sins – sins which demean us and depress our spirits. For this reason, Yom Kippur is a Yom Tov, which deserves to be celebrated with food and drink. However, while repenting, open joy is inappropriate, as one of the main components of repentance is sorrow and regret – which is why Yom Kippur is a fast day. We are therefore commanded to give physical expression to the joy of the mitzva of repentance on Erev Yom Kippur. Thus, we can repent completely on Yom Kippur, despite the somber nature of the day (Sha’arei Teshuva 4:8; R. Moshe Cordovero).
This mitzva entails eating and drinking more than usual. Some say that on Erev Yom Kippur one should eat two days’ worth of food. Le–khatḥila, one should eat at least one bread-based meal; many have two such meals. (See MB 608:18; Kaf Ha-ḥayim 604:2.) Arizal says that eating for the sake of heaven on Erev Yom Kippur allows a person to make up for any spiritual imperfection connected with his eating during the year. This eating is so important that one should study less Torah to fulfill this mitzva maximally (MA 604).
Even though there is a mitzva to eat and drink large amounts, one should eat light, easy to digest foods, rather than heavy ones. We must also be careful not to get drunk, as prayer requires sobriety (SA 608:4; MB ad loc. 18).
The primary mitzva is to eat during the day, not the preceding night (Vilna Gaon; AHS 604:5). Still, several poskim write that eating at night is something of a mitzva as well (Baḥ; Birkei Yosef).
Even if one is exempt from fasting (e.g., one is dangerously ill), there is a mitzva to eat on Erev Yom Kippur, to express the joy of the day and the joy of the mitzva (Ketav Sofer OḤ §112). If he does not have the strength to eat very much, he should at least be careful not to fast, as fasting on Erev Yom Kippur is prohibited, even for those trying to neutralize a bad dream (Rema 604:1). Someone who eats even a small amount of food (the volume of a kotevet, a large date) or drinks a small amount of liquid (a cheek-full) is no longer considered to be fasting (Minḥat Ḥinukh 313:15).