The passuk statesועניתם את נפשותיכם בתשעה לחודש בערב and you shall afflict yourselves on the 9th of the month in the evening. The Gemara brings, from here we derive that whoever eats on erev Yom Kippur is as though he fasted on the 9th day and the 10th day. The Magen Avraham says it is a mitzvah d’oraisa to eat and drink on erev Yom Kippur. Also the Gemara brings, erev Yom Kippur is one of the two times a year a person is supposed to lessen his learning in order to be able to eat. Why is there such a unique mitzvah to eat on erev Yom Kippur?
Rabbeinu Yona explains just as any Yom Tov has a mitzvah to eat a seudah on its day so too Yom Kippur has a mitzvah to eat a seudah. This seudah is eaten the day before, since we need cannot eat on Yom Kippur. Rabbeinu Yona brings another explanation, we eat a seudah to show our extreme happiness of arriving at this long awaited time of becoming forgiven for our sins. A person who sinned is disturbed that he sinned and has pain, he looks forward to Yom Kippur in order to atone for his sins. By rejoicing with a seudah it testifies his feeling of anticipation to be atoned and be rid of his sins.
The Rabbeiu Yona brings another explanation, we eat and drink in order so that we should have strength to be able to fast well. The Rosh adds that Hashem commands us to prepare on erev Yom Kippur to strengthen ourselves by eating and drinking in order to be able to fast well the next day. This is as a person has a son who is beloved and cherished and he commands him to fast on one day. He then commands for his son to be given food and drink, in order to be able to deal with fasting. So to Hashem shows His special love to Yisrael, and commands us on one day a year to fast for our benefit, in order to be able to receive atonement for our sins. Together with this, Hashem also commands us to eat and drink in order to be able to manage well with the fasting.
The Karbon Nesanel brings b’shem the Zohar that on the 9th day, on erev Yom Kippur, we afflict our soul by eating, since the soul wants to be completely ruchni without physical pleasures. And on the 10th day, on Yom Kippur, we afflict our body, the nefesh behamis, by not eating and drinking. The affliction of these two days include our entire being in which both the body and the soul join to experience a complete affliction.
The Tur brings a mashal with the ruler of a city who commanded his servant to buy him fish. He only found one fish for which he was ready to pay a gold coin. There was a Jewish tailor there who also wanted to buy this fish and bid for 5, and the tailor bought it. The servant returned to the ruler and told over what happened and the ruler sent after the tailor. The ruler asked to him, “What is your work?” He responded, “A tailor.” He asked, “Why did you buy the fish for 5 gold coins? You took it away from my servant whom I sent to buy it.” He responded, “How can I not buy it for even 10 gold coins, to be able to eat it on this day which Hashem commanded to eat and drink, reflecting that we trust in Hashem that He will forgive us for our sins.” The ruler responded, “If so, you did well,” freed him and the tailor returned in peace.
May our eating and drinking on erev Yom Kippur reflect our excitement of being atoned!