Yom Kippur, the Day of atonement, is the most sacred of the Jewish holidays. It is regarded as the "Sabbath of Sabbaths."
By Yom Kippur the 40 days of repentance, that begin with the first of Elul, have passed. On Rosh Hashanah
the God Almighty has judged most of mankind and has recorded his judgment in the Book of Life. But he has given a 10 day reprieve.
On Yom Kippur these 10 days of reprieve ends and the Book of Life is closed and sealed. Those
who have repented for their sins are granted a good and happy New Year.
Since Yom Kippur is the day to ask forgiveness for promises broken to God , the day before is reserved for asking forgiveness for broken promises between people, as God cannot forgive broken promises between people.