Yom Kippur is the day of atonement on the Jewish calendar, it concludes the High holidays which started 10 days earlier with Rosh Hashanah. Yom Kippur is considered the holiest day of the year, where in Israel the country virtually stops, including the airport. The day is reserved for intense prayer, fasting, and has a somber feeling. It is called the Shabbat of Shabbats in the Torah and the restrictions of Yom Kippur are similar to those of Shabbat, plus a few extra.
We say that on Yom Kippur the entire world is forgiven for any wrongs they have done in the year. When the sun sets everyone starts on a completely fresh slate for the upcoming year. The day contains numerous significant events in history, and in Jewish philosophy it teaches that the day itself contains the holiness, and we have an opportunity to tap into it.
There are several restrictions on the day of Yom Kippur. Fasting is the most well-known, and for 25 hours people refrain from food, beverage, even washing hands unnecessarily and some brushing their teeth. The day of Yom Kippur is filled with such a high level of holiness that we attempt to distance ourselves from physical things if it will not be a danger to our health. Other restrictions are people do not wear leather shoes, using creams or oils, and avoiding intimate relationships with our spouses.
There is a universal custom to submerge in a mikvah before Yom Kippur starts. The Mikvah is a symbol of purity in the Jewish faith and allows us to connect with our inner soul which is pure.
Individual and national responsibility are two major themes of Yom Kippur. Any wrongs that have been committed against fellow man, we can only be forgiven in heaven if we first ask for forgiveness from that individual. Any wrongs that are between man and his creator can be forgiven on the day of Yom Kippur as well. These ideas bring the nation of Israel closer together in a sense of community and working towards the same goals to improve ourselves and the world.