The Torah tells us about Yom Kippur:
This shall be an eternal law for you: Each year on the 10th day of the 7th month you must afflict yourselves and not do any melachah. This is true for both the native born and the convert to Judaism who joins you. This is because on this day you shall have all your sins atoned, so that you will be cleansed. Before G-d you will be cleansed of all your sins. It is a Sabbath of Sabbaths to you, and a day upon which you must afflict yourselves. This is a law for all time.
— Leviticus 16:29-31
Well, now, there is a lot to explain about those verses, so we’ll take them one at a time.
The seventh month is Tishrei, and the tenth day of Tishrei is the day we call Yom Kippur, the Day of Atonement. Actually, the Torah calls it Yom Kippurim, a Day of Atonements (Leviticus 23:28, et al). Apparently, we can achieve atonement for many things on that one day.
Affliction does not mean that you should invent creative means of torturing yourself. The Torah outlines five forms of affliction for Yom Kippur. On Yom Kippur it is forbidden to:
- Eat or drink
- bathe or wash
- wear leather shoes, leather sandals or any other leather footgear
- have marital relations
- anoint oneself
(Annointing refers to rubbing or applying on one’s body any substance — such as oil, soap, alcohol, hair tonic, cream, ointment, perfume, etc., unless it’s for medical reasons).
Those are the basic categories of “affliction” required on Yom Kippur.
What is melachah? Well, that is a complex thing, and that needs mountains of explanations. The Torah forbids on the Sabbath and Yom Kippur 39 categories of creative activities. On most Holidays, some of those acts are permitted, but not on Yom Kippur.
To return to the verse we are studying:
“For on this day you shall have all your sins atoned . . .” On Yom Kippur, we are guaranteed to have our sins atoned, if we repent. You can’t get a better deal anywhere! Hashem personally forgives each and every sin you have committed, if you confess the sin, and determine to try to never commit it again. We are cleansed and purified, and we can become as if we had never sinned at all!
The Talmud has a beautiful passage about this.
Rabbi Akiva taught: Fortunate are you, Children of Israel! Before Whom are you cleansed, Who cleanses you? Your Father in heaven! As it says, “I will sprinkle upon you clean water, and you will be cleansed of all your impurities . . .” (Ezekiel 36:25) Furthermore, it says “G-d is Israel’s hope.” [The word “hope” is a homonym of the word “mikvah,” the ritual
purification pool.] Just as the mikvah cleanses the impure, so does the Holy One, blessed is He, cleanse the Children of Israel.
— Mishnah Yoma 8:9
Because on Yom Kippur we are guaranteed to be forgiven if we repent.
And that is the primary Mitzvah of that day: to repent.
And if we repent, we are forgiven. That’s all there is to it.