Yom Kippur, starts at day 9 and ends on 10 of Tishrei, 5772 (Jewish Year).
In 2011 starts at sundown on Friday night 7 October. (Kol Nidre), and lasts all day Saturday 8 October, 2011 until sundown.
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.
- As I noted in Days of Awe, 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.
- Most of the holiday is spent in the synagogue, in prayer. 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.
- The liturgy for Yom Kippur is much more extensive than for any other day of the year. This prayer book is called the 'Machzor'. The evening service that begins Yom Kippur is commonly known as Kol Nidre, named for the prayer that begins the service. "Kol nidre" means "all vows," and in this prayer, we ask G-d to annul all personal vows we may make in the next year. It refers only to vows between the person making them and G-d, such as "If I pass this test, I'll pray every day for the next 6 months!"
- There are many additions to the regular liturgy. Perhaps the most important addition is the confession of the sins of the community, which is inserted into the Shemoneh Esrei (Amidah) prayer, all sins are confessed in the plural (we have done this, we have done that), emphasizing communal responsibility for sins.
After Yom Kippur, one should begin preparing for the next holiday, Sukkot, which begins five days later.