Leviticus 23:27

But on the tenth day of this seventh month is the Day of Atonement; a holy convocation shall it be to you, and you shall humble your souls, (abstaining) from food, and from drink, and from the use of the bath, and from anointing, and the use of the bed, and from sandals; and you shall offer an oblation before the Lord,

Scripture says However concerning the holiday on the tenth because normally a holiday is a happy day. For example, in Ezra’s book we see the phrase “go your way, eat sumptuously, and drink sweet beverages” [Nehemiah 8:10].

you shall afflict yourselves explained above [comment on 16:29].

ACH’ (BUT) ON THE TENTH DAY OF THIS SEVENTH MONTH IS THE DAY OF ATONEMENT. “All expressions in the Torah of ach (but) and rak (only) indicate limiting qualifications, [and here the word ach indicates that the Day of Atonement] effects atonement ‘only’ for those who repent, but it does not effect atonement for those who do not repent.” This is Rashi’s language on the basis of the words of our Rabbis. And if so, the explanation of the verse is that on the first day of this month [Tishri] there will be for all of you a day of memorial of ‘t’ruah [a blowing of the quavering sound which signifies judgment], for you will all be judged then before Him, but for some of you [i.e., those who repent] there will be on the tenth day of this month a day of atonement. Thus the meaning of the word ach is like that of bilvad (“only,” “alone”). Similarly, Now therefore forgive, I pray thee, my son ‘ach’ (only) this once. So also, ‘harak ach b’Moshe’ means “has He indeed spoken ‘only’ with Moses?” Likewise, ‘Ach’ on the fifteenth day of the seventh month means “only on the fifteenth day [of Tishri] ye shall keep the feast of the Eternal seven days, not consecutively,” for the Festival-offering [brought by the individual] does not override the Sabbath [and therefore it cannot mean seven days consecutively]. In the same way you are to explain [the word ach] in all the commandments, in accordance with the tradition of our Rabbis. Similarly, ‘ach’ he is torn in pieces means [that Jacob said] that nothing else [could have happened to Joseph] except that he was torn in pieces. So also, For the children of Israel and the children of Judah ‘ach’ have done that which was evil in My sight from their youth; for the children of Israel have ‘ach’ provoked Me, means that they have done nothing else.
But by way of the plain meaning of Scripture the word ach means achein (“surely”), [a word which is used] to authenticate something, [as in the following expressions]: ‘achein’ (surely) the thing is known; ‘achein’ (surely) ye shall die like men. And here it means, “Surely on the tenth day of the seventh month is the day of atonement,” the expression constituting a [Divine] assurance on the authenticity of the matter, Scripture stating: “On the first day of the month is the day of judgment, but indeed on the tenth of the month is the day of atonement, therefore you shall afflict your souls, and you shall do no manner of work.” Similarly, ‘ach’ (surely) thou art my bone and my flesh; ‘ach’ (surely) it is the king of Israel; ‘ach’ (surely) G-d is good to Israel, even to such as are pure in heart. So also is the meaning of the verse, ‘ach’ ye shall keep My Sabbaths which declares: “Now I have commanded you concerning the work of the Tabernacle, but surely My Sabbaths you shall keep forever.” So also in all cases [where the word ach is mentioned] it may be explained to you in this way, if you will understand them.

'אך בעשור וגו, on the other festival days preparation of food had been permitted and only מלאכת עבודה had been forbidden. Not so on Yom Kippur, a day of self denial when all manner of work is prohibited just as on the Sabbath

והקרבתם אשה, as already mentioned in chapter 16 and again in Numbers 29,8-11.

אך 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.]

אך בעשור לחודש, “but on the tenth of the month, etc.;” why do we need the word: “אך?” Seeing that all the other festival days are known as מקרא קודש, “holy convocation,” i.e. days for eating drinking and general enjoyment, whereas this is a day on which all these things are forbidden, the word אך introduces Yom Kippur as a contrast by letting us know that it is no less of a festival, מועד, although the emphasis is the atonement that is granted to the entire nation on that day. It is still called מקרא קודש on account of the day being honoured by the sacrifices being offered and by the abstaining from the normal weekday activities. On that day, we subject ourselves to afflictions and at the end of the day we all feel even better than at the end of any of the other days called מקרא קודש.