Yom Kippur, the last of the Days of Awe, is observed on the tenth of Tishri. While Shabbat is the holiest of days, it is only human nature to regard Yom Kippur, which occurs only once a year as opposed to once a week, as something very special and out of the ordinary. This is why you will see some Jews, who never attend a Shabbat service during the rest of the year, go to synagogue on Yom Kippur.
Yom Kippur is the “Day of Atonement.” It is a day to atone for the sins of the prior year. Yom Kippur is sometimes referred to as the “Sabbath of Sabbaths” and has been an integral part of Judaism for thousands of years.
Prohibitions on Yom Kippur
Almost everyone, even non-Jews, knows that Yom Kippur is a day when Jews are forbidden to eat and drink. In fact, fasting is only one of five prohibitions that must be obeyed. Following the rules governing fasting is very simple. You don't eat and you don't drink! The fast commences before sunset on the evening of Yom Kippur and ends after nightfall the next day.
There is no need to have a reason to fast. It is a mitzvah from God that appears in the Torah. However, many Jews need explanations to justify the practices of Judaism, and many rationales have been offered in this regard.
For one, refraining from consuming food or liquid is a concrete expression of the gravity of the day. It helps each person attain the state of mind required to focus on the spiritual. Furthermore, fasting manifests a form of self-mastery over bodily needs. Another more socially conscious justification states that by fasting, people can identify more readily with the poor and the hungry.
Any of these reasons, or any one that is important and meaningful to you, will do. The point is that fasting is fundamental to the observance of Yom Kippur.
Jews need to observe the mitzvah to fast as long as it does not pose a physical threat. Children under the age of nine and women in childbirth (that is, from the time the labor commences to three days following the birth) are absolutely not permitted to fast. Older children, not yet bar or bat mitzvah, and women from the third to the seventh day after childbirth, are permitted to fast, but should resume eating or drinking if they feel the need.
In addition to fasting, the following prohibitions apply to observing Yom Kippur:
No washing or bathing
No using creams and oils (a prohibition that extends to deodorants and cosmetics)
No sexual relations
No wearing leather shoes
One reason for not wearing leather shoes is the incongruity of deriving a benefit from the slaying of one of God's creatures while praying and beseeching God for a long life. This proscription might explain why it's not uncommon to see men wearing formal suits and canvas sneakers on Yom Kippur.