(The below is a slight adaptation of my sermon for Yom Kippur morning; it argues that concerns of economic and social inequality are perhaps more rooted in Yom Kippur than we typically realize, and as such may be interesting to people looking to think about how social justice appears in the textual tradition surrounding Yom Kippur.) 
Can you wish someone a “Happy Yom Kippur”? Sometimes I’m wished a “Happy Yahm Kippur” by a well-meaning, non-Jewish friend, but of course that’s not right. Yom Kippur is the Day of Atonement, a day of fasting, a day of solemnity. These are, after all, the High Holy Days—not the High Happy Days.
And yet, in the Mishnah (Taanit 4:8), the basic text of rabbinic law, we find: “Rabban Shimon ben Gamliel said, ‘There were no festivals (yamim tovim) in Israel greater than the fifteenth of Av’”—that’s for another sermon—“‘and the Day of Atonement (Yom Hakippurim).’” And he goes on to describe a day on which the maidens of Jerusalem would “go out dressed in white garments… and dance in the vineyards,” flirting with the young men. This is Yom Kippur as Junior Prom.
So is Yom Kippur festive or solemn? A day for socializing, or for contemplation? For parties, or for penitence? That’s what I want to discuss. To sharpen the problem, look at the Torah reading for Yom Kippur morning. Here is how Leviticus 16 characterizes the day:

In the seventh month, on the tenth day of the month, you shall afflict yourselves (t’anu at nafshoteikhem); and you shall do no manner of work, neither the citizen nor the alien who resides among you. For on this day atonement shall be made for you to cleanse you of all your sins; you shall be clean before God. It shall be a Sabbath of Sabbaths for you, and you shall afflict yourselves; it is a law for all time. The priest… shall make expiation. He shall put on the linen vestments, the sacral vestments. He shall purge the innermost Shrine; he shall purge the Tent of Meeting and the altar; and he shall make expiation for the priests and for all the people of the congregation… (Lev 16:29-34).

“Sins,” “atonement,” “expiation,” “purge,” “afflict”: the mood here is obviously much darker than in the Mishnah. “Inui,” or “affliction,” is the same root used for Egyptian slavery (vayanunu, they afflicted us). In modern Hebrew, it means “to torture.” Torture yourselves. What could Rabban Shimon ben Gamliel have been thinking?
Part of the answer is that although Leviticus 16 is the central biblical text about Yom Kippur, it’s not the only one. Later in Leviticus, in Chapter 25, the holiday appears again, with a different valence. Here are the relevant verses:

Count off seven sabbath years—seven times seven years—so that the seven sabbath years amount to a period of forty-nine years. Then sound the trumpet everywhere on the tenth day of the seventh month; on the Day of Atonement sound the trumpet throughout your land. Consecrate the fiftieth year and proclaim liberty throughout the land to all its inhabitants. It shall be a jubilee for you; each of you is to return to your family property and to your own clan… If any of your fellow Israelites become poor and sell themselves to you, do not make them work as slaves. They should be hired, temporary workers among you until the Year of Jubilee. Then they and their children are to be released, and they will go back to their clans and to their ancestral property (Lev 25:8-10, 39-41).

The Jubilee year (sh’nat hayovel) is a massive economic reset for Israelite society. Slaves go free, lands revert to their ancestral owners, the whole society returns to basic equality. The mood in this text is much happier: not for nothing is the English word “Jubilee” is related to “jubilation.” It is a day of release, of shared prosperity, of freedom.
To be sure, the Jubilee occurs only once in fifty years. This is not every Yom Kippur. Still, you should ask, why does the Jubilee start on Yom Kippur at all? The logical start-point of a Jubilee-year would be, well, the New Year. And Rosh Hashanah is indeed the day of blowing the shofar (yom t’ruah) elsewhere in the Torah—not Yom Kippur. So why start ten days late? The Talmud was bothered by that same problem, and here’s what Rabbi Yishmael, son of Rabbi Yoḥanan ben Beroka suggests:

From the New Year until the Day of Atonement, the slaves were not released to their homes, but nor were they enslaved to their masters. Rather they ate and drank, and there were crowns on their heads. When the Day of Atonement arrives, the Court blew the horn, and the slaves were released to their homes, and the fields reverted to their owners (Bavli Rosh Hashanah 8b).

I think this passage is important for two reasons.
First, it shows that the Rabbis read the Jubilee as a celebratory moment of feasting and revelry. In this context, the risqué idea of a Yom Kippur dance party makes perfect sense. This is a season of social inversions, of mixed dancing and slaves wearing crowns. The Jubilee Yom Kippur is not just a festival; it’s a carnival.
Second, the Talmud also explains why the Jubilee and Yom Kippur go together. The ten days between Rosh Hashanah and Yom Kippur are the Ten Days of Repentance (Aseret Y’mei Hateshuvah). And that’s exactly what’s happening here. The slaves are returning to freedom. The society, long used to hierarchical oppression, is recalibrating, learning that everybody is equal again. (The masters, of course, may well feel like they are afflicting themselves; they are certainly being purified of their sins.) The Jubilee occurs on Yom Kippur because the Jubilee is the ultimate form of collective return, collective teshuvah.
The tradition I am tracing—of Yom Kippur as a joyous day of social equality—also shows up, and helps make sense of, the Haftarah for Yom Kippur. In it, God tells Isaiah to address the Israelites: “Raise your voice like a horn. Tell my people their rebellion and the house of Jacob their sins.” Isaiah’s prophecy here is a substitute-shofar blast, calling the people to repent but also, as we’ll see, to a Jubilee. The Israelites feel cheated by God: “‘Why have we fasted,’ they say, ‘and you have not seen it? We afflicted ourselves, and you have not noticed!’” They’ve observed Leviticus 16; what’s gone wrong? God explains: “Yet on the day of your fasting, you do as you please and exploit all your workers.” They’ve got the atonement ritual of Leviticus 16 down; but they’ve failed the test of the Jubilee.
God proposes a radical revision of Yom Kippur:

Is this the fast I have chosen, only a day for people to afflict themselves? Is it only for bowing one’s head like a reed and for lying in sackcloth and ashes? Is that what you call a fast, a day acceptable to God? Is not this fast I have chosen: to loose the chains of injustice and untie the cords of the yoke, to set the oppressed free and break every yoke? Is it not to share your food with the hungry and to provide the poor wanderer with shelter—when you see the naked, to clothe them, and not to turn away from your own flesh and blood?… Then you will call, and God will answer; you will cry for help, and God will say: Here am I. If you do away with the yoke of oppression, with the pointing finger and malicious talk, and if you spend yourselves on behalf of the hungry and satisfy the needs of the oppressed, then your light will rise in the darkness… (Isaiah 58:1, 3-11).

The first point to make about this is that we usually read Isaiah too weakly. He is not saying that the penitential ritual is good but insufficient, that it needs to be complemented by social justice. He is rather, as Rabbi Shlomo Zuckier points out, turning Leviticus 16 on its head. You think Yom Kippur for fasting? Wrong. It’s for feeding people! You think it’s for degrading clothing? Wrong. It’s for clothing the naked. You think it’s for inui, affliction? Wrong. It’s for the oni, the poor. And indeed, the promise of Yom Kippur here is not atonement, but joy: “If you keep your feet from breaking the Sabbath”—remember, Yom Kippur is the Sabbath of Sabbaths—“and from doing as you please on my holy day, if you call the Sabbath a delight and God’s holy day honorable… then you will find your joy in God.” Isaiah is not supplementing ritual with piety; he is replacing penitential ritual with social equality and joy—that is, with Jubilee.
Here’s the second point about Isaiah. We often read this Haftarah as a critique of a Temple ritual that has lost touch with ethics. Here comes the moral prophet, turning from ritual to ethics. But now that we’ve seen the Jubilee tradition, we can do better. Isaiah isn’t departing from the tradition of Yom Kippur. He’s reasserting the importance of the Jubilee tradition, an alternative model of Yom Kippur that has always been there in Jewish tradition, right alongside the Temple solemnity. The great Bible scholar Jacob Milgrom suggests that underlying Yom Kippur was a folk holiday, a yearly, informal ritual of social inversion (that is, a Jubilee) that the Temple ritual controls, formalizes, and re-channels into religious worship. Isaiah is unleashing that folk-tradition against a corrupt economic elite.
Where does that leave us? Well, first, these texts help understand our own conflicting practices of Yom Kippur. Of course we’re sad and penitent today, and much of the liturgy expresses that: “who shall live and who shall die,” “for the sin we’ve sinned before you” etc. But Yom Kippur remains a festive day too. We are encouraged to eat a hearty pre-fast meal (Bavli Yoma 81b), and we don’t need to be encouraged to eat a serious break-fast. And these texts say: that is not an accident. Yom Kippur is, strangely, at once both a fast and a feast.
But second, they say something tougher too. Yom Kippur is not just a party: it is a party about social equality. For us to feast, everyone in our society has to be able to feast too. The Jubilee requires a deep critique of a society where some have great wealth and others are working for too little. The Jubilee requires returning to a more equal world, where everyone has enough and no one is a slave. The Jubilee requires not just individual regret and introspection, but the creation of a just and caring society. A Happy Yom Kippur, and a may we be sealed for life: G’mar Chatima Tova.