Day of Atonement

Leviticus 16

29 And this shall be a statute for ever unto you: that in the seventh month, on the tenth day of the month, ye shall afflict your souls, and do no work at all, whether it be one of your own country, or a stranger that sojourneth among you:

30 For on that day shall the priest make an atonement for you, to cleanse you, that ye may be clean from all your sins before the Lord.

Yom Kippur: The Day Of Atonement

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.

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.

This day is, essentially, your last appeal, your last chance to change the judgment, to demonstrate your repentance and make amends.

The services end at nightfall, with the blowing of the tekiah gedolah, a long blast on the shofar.

(Source: Judaism 101)