Today is the start of Yom Kippur

Yom Kippur, the Day of Atonement, is the most sacred of the Jewish holidays, the "Sabbath of Sabbaths."By Yom Kippur the 40 days of repentance, that begin with the first of Elul, have passed.

On Rosh Hashanah G-d has judged most of mankind and has recorded his judgement in the Book of Life. But he has given a 10 day reprieve.On Yom Kippur the Book of Life is closed and sealed. Those that 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 G-d, the day before is reserved for asking forgiveness for broken promises between people, as G-d cannot forgive broken promises between people.

Yom Kippur is a day of "NOT" doing. The is no blowing of the Shofar and Jews may not eat or drink, as fasting is the rule. It is believed that to fast on Yom Kippur is to emulate the angels in heaven, who do not eat, drink, or wash. The Five Prohibitions of Yom Kippur

Eating and drinking
Anointing with perfumes or lotions
Marital relations
Washing
Wearing leather shoes