Leviticus 23:26-28What had Moses to atone for? It was actually his followers, rather than Moses, who had been busy talking with the Burning Bush, but Moses had taken their case to God. He might have destroyed them had Moses not intervened. Moses' followers had been worshiping the Golden Calf and ignoring the God of the Hebrews.
And the LORD spake unto Moses, saying, Also on the tenth day of this seventh month there shall be a day of atonement: it shall be an holy convocation unto you; and ye shall afflict your souls, and offer an offering made by fire unto the LORD. And ye shall do no work in that same day: for it is a day of atonement, to make an atonement for you before the LORD your God.
Yom Kippur continues to be celebrated as the Day of Atonement, 8 days after the Jewish New Year holiday of Rosh Hashanah. Chaim.org says that the heavenly court gates open on Rosh Hashanah and close when the Shofar (ram's horn) is blown at the end of the Yom Kippur service. The court then judges the deeds of mankind.
Yom Kippur, which is preceded by a large family meal, starts at nightfall and ends the following day at nightfall.
See the article on Yom Kippur at About.com.