The commandment to keep Yom Kippur (the tenth day of Tishrei) as a day of rest and solemnity teaches that we are commanded to begin on the ninth day of Tishrei, and continue from evening to evening (see Vayikra 23:32). The Gemara on our daf quotes Rabbi Yishmael as learning the rule of tosefet Yom ha-kippurim – beginning the holiday early and completing it late – from this passage, a rule that is then extended to Shabbat and Yom Tov, as well.
Hiyya bar Rav of Difti taught the following baraita: The verse states: “And you shall afflict your souls on the ninth of the month.” Is the fasting on the ninth? But isn’t the fasting on Yom Kippur on the tenth of Tishrei? Rather, this verse comes to teach you: Whoever eats and drinks on the ninth, thereby preparing himself for the fast on the next day, the verse ascribes him credit as though he fasted on both the ninth and the tenth.
This is generally understood to mean that there is a special mitzva to eat on the day before Yom Kippur.
Several explanations are given for this law. Rashi and the Meiri suggest that since there is a mitzva to fast on the tenth, someone who spends the day before preparing for that mitzva is given credit for the preparation. The Eliya Rabba (Rabbi Elijah Shapira’s gloss on the Shulhan Arukh) suggests otherwise. According to him, someone who eats a lot the day before the fast has a harder time refraining from eating on the fast day, therefore the person who spends the ninth of Tishrei eating is credited for having additional inuy (deprivation). Others point out that Yom Kippur is a holiday, a day on which we really should be eating and drinking. Since we cannot eat and drink on Yom Kippur, we “make up” for it on erev Yom Kippur. Finally, some explain that this is preparation for the mitzva of expressing regret and asking for forgiveness. Since someone who is well-fed is less likely to be irritable and get into disagreements, we are commanded to put ourselves into such a position so that we will be better suited to be remorseful and apologize.