There is a mitzvah to eat on Erev Yom Kippur. The pasuk says that we should do ennui to our soul on the ninth day of Tishrei. The Gemara, in Yuma 81b, explains that the pasuk cannot be referring to the ninth day because we know from other pesukim that the ennui is on the tenth of the month. Therefore the Gemara explains that the pasuk is teaching us that whoever eats on the ninth day is considered to have fasted on the ninth and tenth days.
There is a dispute as to the nature of this obligation. The Magen Avraham (604:1) discusses whether the obligation applies only during the day or also on the night before Yom Kippur. The Vilna Gaon there says that the main obligation is only during the day.
This dispute may depend on the different reasons for the obligation to eat on Erev Yom Kippur. Rabbeinu Yona, in Sha’arei Teshuvah (shar 4:8-10) brings three different reasons for the obligation to eat on Erev Yom Kippur. One reason is due to the simcha of the kapparah of Yom Kippur. A second reason is that it is a seudas Yom Tov. Tosafos, in Chullin 133a, also says explicitly that Erev Yom Kippur is a Yom Tov and therefore we eat a seudah. The third reason is so that we will have strength to fast on Yom Kippur. Rashi, in Yuma 81b, states this reason as well.
The first two reasons would seem to suggest that the obligation to eat on Erev Yom Kippur is similar to that of a Yom Tov, and would thus apply even on the night before Yom Kippur. However, the last reason would only apply to the day before Yom Kippur.
Based on this, there can also be another halachic ramification. If one is sick and must eat on Yom Kippur, does he have an obligation to eat on Erev Yom Kippur? If the reason why we eat on Erev Yom Kippur is because it is a Yom Tov, then even one unable to fast on Yom Kippur should still be obligated to eat on Erev Yom Kippur. But if the obligation to eat is to give us strength to fast on Yom Kippur, then one who will not be fasting should not have an obligation to eat on Erev Yom Kippur.
The Rama (604:1) says that one may not fast – even a ta’anis chalom (a fast when one experiences a disturbing dream) on Erev Yom Kippur. This ruling is perplexing, as generally we allow one to fast a ta’anis chalom, e.g. on Shabbos, Yom Tov and Purim.
If we understand that the obligation to eat on Erev Yom Kippur is similar to that of Shabbos and Yom Tov, then this ruling is indeed difficult. However, if the reason for the obligation is to ensure that we will be able to daven with kavanah during the fast of Yom Kippur, we can then differentiate between the obligation to eat on Erev Yom Kippur and to do the same on a regular Shabbos and Yom Tov. On Shabbos and Yom Tov there is no obligation, per se, to eat; the obligation is oneg or simcha, which by and large manifests itself by means of eating. Yet if one will feel more comfortable by not eating, he has no obligation to eat. Thus if one wishes to fast a ta’anis chalom, he may do so on Shabbos and Yom Tov. But on Erev Yom Kippur the specific obligation is to eat; therefore, one may not choose to instead fast.
Another corollary between these two reasons for the obligation to eat on Erev Yom Kippur is the question of whether women are included in this obligation. Rav Akiva Eiger (Teshuvos 15) discusses this question and whether women are excluded, since the question arises as to whether it is a mitzvas assei she’hazeman gramma (time-sensitive mitzvah). If the obligation to eat is because it is similar to a Yom Tov, women should then be exempt. Then again, if the reason is because we can better daven and do teshuvah on Yom Kippur, even women would be obligated to eat on Erev Yom Kippur.
The Ksav Sofer (114) says that if a woman knows that she will not be fasting on Yom Kippur, she should have no obligation to eat on Erev Yom Kippur. This is because, as we explained earlier, the only reason why one who knows that he will be eating on Yom Kippur and should be obligated to eat on Erev Yom Kippur is that it is a Yom Tov. However, a woman is not obligated to eat on Erev Yom Kippur if it is a Yom Tov. A woman would only be obligated to eat if the reason is to ensure strength for the following day’s fast. But if a woman is not going to fast on the next day, there is no reason for her to eat on Erev Yom Kippur.