Also known as Instant Lent so the old joke went. This holiday begins tonight and will end at sunset tomorrow.
Yom Kippur is the Holiest Day of the Jewish Year. From the Jewish Virtual Library:
Yom Kippur is probably the most important holiday of the Jewish year. Many Jews who do not observe any other Jewish custom will refrain from work, fast and/or attend synagogue services on this day. Yom Kippur occurs on the 10th day of Tishri. The holiday is instituted at Leviticus 23:26 et seq.The customs of Yom Kippur are:
The name “Yom Kippur” means “Day of Atonement,” and that pretty much explains what the holiday is. It is a day set aside to “afflict the soul,” to atone for the sins of the past year. In Days of Awe, I mentioned the “books” in which G-d inscribes all of our names. On Yom Kippur, the judgment entered in these books is sealed. This day is, essentially, your last appeal, your last chance to change the judgment, to demonstrate your repentance and make amends.
Yom Kippur atones only for sins between man and G-d, not for sins against another person. To atone for sins against another person, you must first seek reconciliation with that person, righting the wrongs you committed against them if possible. That must all be done before Yom Kippur.
Yom Kippur is a complete Sabbath; no work can be performed on that day. It is well-known that you are supposed to refrain from eating and drinking (even water) on Yom Kippur. It is a complete, 25-hour fast beginning before sunset on the evening before Yom Kippur and ending after nightfall on the day of Yom Kippur. The Talmud also specifies additional restrictions that are less well-known: washing and bathing, anointing one's body (with cosmetics, deodorants, etc.), wearing leather shoes (Orthodox Jews routinely wear canvas sneakers under their dress clothes on Yom Kippur), and engaging in sexual relations are all prohibited on Yom Kippur.
As always, any of these restrictions can be lifted where a threat to life or health is involved. In fact, children under the age of nine and women in childbirth (from the time labor begins until three days after birth) are not permitted to fast, even if they want to. Older children and women from the third to the seventh day after childbirth are permitted to fast, but are permitted to break the fast if they feel the need to do so. People with other illnesses should consult a physician and a rabbi for advice.
Most of the holiday is spent in the synagogue, in prayer. In Orthodox synagogues, services begin early in the morning (8 or 9 AM) and continue until about 3 PM. People then usually go home for an afternoon nap and return around 5 or 6 PM for the afternoon and evening services, which continue until nightfall. The services end at nightfall, with the blowing of the tekiah gedolah, a long blast on the shofar
It is customary to wear white on the holiday, which symbolizes purity and calls to mind the promise that our sins shall be made as white as snow (Is. 1:18). Some people wear a kittel, the white robe in which the dead are buried.
You can discover more about the customs and practices of Yom Kippur here.
Finally, in the waning hours of the day, we reach the climax of the day: the fifth prayer, the Neilah ("locking") prayer. The gates of heaven, which were open all day, will now be closed-with us on the inside. During this prayer we have the ability to access the most essential level of our soul. The Holy Ark remains open throughout. The closing Neilah service climaxes in the resounding cries of "Hear O Israel . . . G‑d is one." Then joy erupts in song and dance (a Chabad custom is to sing the lively "Napoleon's March"), followed by a single blast of the shofar, and the proclamation, "Next year in Jerusalem."
After the fast we partake of a festive after-fast meal, making the evening after Yom Kippur a yom tov (festival) in its own right. We immediately begin to look forward to the next holiday and its special mitzvah: the construction of the sukkah.
May you have an easy fast.