Sforno on Leviticus 23:27/ספורנו על ויקרא כ״ג:כ״ז

1אך בעשור לחודש השביעי, even though on all the other days described in this chapter as מקראי קודש it is proper to express our joy on these days by means of food and drink, as we know from Nechemyah 8,10 where Ezra asks the people on New Year’s day אכלו משמנים ושתו ממתקים, “eat choice foods and drink sweet drinks,” the tenth of this month the day known as Yom Hakippurim, Day of Atonement, is a day devoted to confession of our sins and a day when a person comes to grips with the sins he has been guilty of. On this day it is not in order to preoccupy oneself with anything other than the quest to obtain G’d’s forgiveness and to achieve expiation of one’s sins. Instead of eating and drinking, the order of the day has been best expressed by Isaiah 58,3 הן ביום צומכם תמצאו חפץ וכל עצביכם תנגושו, “because on your fast day you see to your business and you oppress your labourers!” [the prophet sarcastically chastises the people who, while technically fasting, expect G’d to listen to their requests while they continue in their wicked ways, going only through the motions in their religious observance. Ed.]