INTRODUCTION:

The idea of Fasting on Yom Kippur comes from the Torah:

וְהָ יְתָ ָ֥ ה לָכֶ ֶ֖ם לְ חֻ קַּ ַּ֣ ת עֹולָ ָ֑ם בַּ ח ַּ֣ דֶ ׁש הַַּ֠ שְ בִ יעִַ֠ י בֶֶּֽ עָ שׂ֨ ֹור לַּח ֹ֜ דֶ ׁש תְּ עַ נּ֣ ּו אֶ ת־נַפְּ ש ֹֽׁ תֵ יכֶֶ֗ ם וְכׇל־מְ לָאכָה֙ ל ַּ֣ א תַּ עֲשׂ֔ ּו הֶָּֽ אֶ זְ רָׂ֔ ח וְהַּ ג ֶ֖ר
הַּ גָ ָ֥ר בְ תֹוכְ כֶֶּֽ ם׃

And this shall be to you a law for all time: In the seventh month, on the tenth day of the month, you shall practice self-denial; and you shall do no manner of work, neither the citizen nor the alien who resides among you. (Leviticus 16:29)

Fasting on Yom Kippur is one of several prohibitions that are mentioned in the Talmud:

מַ תְּ נִי׳ יֹום הַּ כִ ּפּורִ ים אָ סּור בַּ אֲכִ ילָה ּובִ ׁשְ תִ יָה ּובִ רְ חִ יצָה ּובְ סִ יכָה ּובִ נְעִ ילַּת הַּ סַּ נְ דָ ל, ּובְ תַּ ׁשְ מִ יׁש הַּ מִ טָ ה. וְהַּ מֶ לְֶך וְהַּ כַּלָה
יִרְ חֲ צּו אֶ ת ּפְ נ יהֶ ם, וְהַּ חַּ יָה תִ נְעֹול אֶ ת הַּ סַּ נְ דָ ל, דִ בְ ר י רַּ בִ י אֱלִ יעֶ זֶר. וַּחֲ כָמִ ים אֹוסְ רִ ין.

MISHNA: On Yom Kippur, the day on which there is a mitzva by Torah law to afflict oneself, it is prohibited to engage in eating and in drinking, and in bathing, and in smearing oil on one’s body, and in wearing [leather] shoes, and in conjugal relations. However, the king, in deference to their eminence, and a new bride within thirty days of marriage, who wishes to look especially attractive at the beginning of the relationship, may wash their faces on Yom Kippur. A woman after childbirth, who is suffering, may wear shoes because going barefoot causes pain. (these allowances) are the statement of Rabbi Eliezer. The Rabbis prohibit these activities for a king, a new bride, and a woman after childbirth. (BT: Yoma 73b: 13)

Later, Maimonides in the Mishneh Torah expands on these prohibitions as a positive mitzvah:

There is another positive commandment on Yom Kippur, to refrain from eating and drinking, as [Leviticus 16:29] states: “You shall afflict your souls.” According to the Oral Tradition, it has been taught: What is meant by afflicting one’s soul? Fasting. Whoever fasts on this day fulfills a positive commandment. Whoever eats or drinks on this day negates the observance of [this] positive commandment and violates a negative commandment, as [ibid. 23:29] states, “Any soul that does not afflict itself will be cut off.” Since the Torah punishes a person who does not fast with karet, we can derive from this that we are forbidden to eat and drink on this day. A person who eats or drinks inadvertently on this day is liable to bring a sin offering of a fixed nature. (Mishneh Torah: Rest of the 10th of Tishrei 2:4)

In another section on Fasts, Maimonides defines other moments for fasting:

‘We should fast and sound the trumpets in the [following] situations of communal distress: because of the distress that the enemies of the Jews cause the Jews, because of [the passage of] an armed [force], because of a plague, because of a wild animal [on a rampage], because of various species of locusts, because of the black blight and the yellow blight, because of falling buildings, because of an epidemic, because of [the loss of our source of] sustenance, and because of rain [or a lack of it]. (Mishneh Torah: Fasts 2:1)

These are other occasions for which Maimonides allows for a communal fast day. Do these help us understand why we fast on Yom Kippur? Are there any connections?

WHAT IF YOU ARE UNABLE TO FAST?:

Judaism allows for medical exemptions for fasting. As demonstrated in this Talmudic passage, anyone who might be injured from fasting is exempt. Rabbi Shimon ben Menasya said: It is stated: “And the children of Israel shall keep Shabbat, to observe Shabbat” (Exodus 31:16). The Torah said: Desecrate one Shabbat on his behalf so he will observe many Shabbatot. Rav Yehuda said that Shmuel said: If I would have been there among those Sages who debated this question, I would have said that my proof is preferable to theirs, as it states: “You shall keep My statutes and My ordinances, which a person shall do and live by them” (Leviticus 18:5), and not that he should die by them. In all circumstances, one must take care not to die as a result of fulfilling the mitzvot. (Yoma 85b:3)

NOT FASTING THIS YEAR?: 

Read this Meditation Before Yom Kippur for One Who Cannot Fast | Reform Judaism

THE NIGHT BEFORE:

Why is the dinner the night before Yom Kippur special? Here are three reasons for this:

1. To demonstrate our joy in the arrival of the Day of Atonement for our sins.
2. To rejoice in the mitzvah to observe the holiday of Yom Kippur.
3. So that we will have the strength to increase our prayers and supplications on Yom Kippur and to focus on planning strategies for repentance.
   (Shaarei Teshuvah 4:8-10, from Torah.org)

SPIRITUAL EXPLANATIONS FOR FASTING:

“On Yom Kippur we seek reconciliation with God and humanity. Repentance (Teshuvah) involves a critical self-assessment of the past year and the resolve to avoid lapses in sensitivity in the future. Teshuvah requires discipline. Our fasting on Yom Kippur demonstrates our willingness to submit to discipline. How can we atone for our excesses toward others unless we can curb appetites which depend on no one but ourselves? To set boundaries for our own conduct in this very private matter is to begin the path toward controlling our public behavior.” (Why Fast on Yom Kippur? | My Jewish Learning)

“When we refrain from indulging our physical appetites for a limited period, in order to devote ourselves for a time more exclusively to demands that rank higher in our hierarchy of values, we are not denying the physical appetites their just place in life; we are simply recognizing the need of putting them in their place.” (Attributed to Rabbi Mordechai Kaplan, from Your Guide to Fasting (or Not) on Yom Kippur | Reform Judaism)

“On Yom Kippur, we rise above the corporeal reality of day-to-day life, focusing solely on the spiritual. This is the day when we get in touch with our inner selves, our angelic side. And just as angels don’t eat or drink, neither do we.” (Menachem Posner, Yom Kippur Fasting – The Torah Source, Spiritual Meaning, and Practical Info – Chabad.org)

“The purpose of fasting is to experience hunger, to acknowledge that we are not just physical beings, but spiritual ones as well.” – Rabbi Jonathan Sacks