*Just like Rosh Hashanah, Yom Kippur is based on the Jewish calendar (the 10th day of Tishrei) and occurs on a different day each year in the standard calendar. In addition, the Jewish day goes from sundown to sundown, not midnight to midnight. Thus, in 2013, Yom Kippur begins at sunset September 13 and ends at nightfall, September 14.
At sunset tonight, Yom Kippur, the Day of Atonement, begins. In the times of the temple, this is the day the high priest would enter in to the Holy of Holies and sacrifice there to make atonement for the people.
In addition, Yom Kippur is the day the scapegoat was released and many other sacrifices were made. (Read Leviticus 16 for more) In both ancient times and now, it is a day of Sabbath rest, fasting, repentance, and prayer.
“This is to be a lasting ordinance for you: On the tenth day of the seventh month you must deny yourselves and not do any work—whether native-born or a foreigner residing among you— because on this day atonement will be made for you, to cleanse you. Then, before the Lord, you will be clean from all your sins. It is a day of sabbath rest, and you must deny yourselves; it is a lasting ordinance.”