Yom Kippur is celebrated on the 10th day of Tishrei, 10 days after Rosh Hashanah. Over time, more & more importance was placed on Yom Kippur as it also became known as the Day of Judgment-the culmination of the 10 days of repentance. To the Rabbis, Yom Kippur marked the climax of the trial of the people; it was when God sealed names in the book of life, or in the book of death.
The Rabbis further abstentions that must be followed on Yom Kippur which included drinking, bathing, eating, wearing leather shoes & sexual relations. Yom Kippur is so important to the Jews that it is sometimes referred to as the Sabbath of Sabbaths & is the only fast day that can fall on Saturday.
The day is spent in Synagogue praying & supplicating to God to be sealed in the book of Life. Yom Kippur is not a sad day; it is a day of reflection &atonement. Fasting is used not as a punishment, but as a cleansing of the body as well as the spirit.
Some Jews stay in the synagogue all day & only take one small break between prayers. Because the day is very important, there are 5 different prayer services instead of the 4 that appear on a regular Sabbath. Yom Kippur ends with a long resonating Shofar explosion followed by the breaking of the fast; this marks the end of the 10 day long period of repentance & reflection.