Yom Kippur on September 30, 2017 celebrates the reconciliation between God and the Jews. This day is the highest Jewish holiday and is observed annually in September or October. On the 10th day after the Jewish New Year's Day Rosh Hashanah God is supposed to seal the judgment from Rosh Hashanah. On the 10th of Tishrei, the 10th of the 7th month of the Jewish calendar, Yom Kippur, the Day of Reconciliation, is traditionally celebrated with Lent and intensive phases of prayer. Believers spend the most time of the day at the synagogue.
Yom Kippur is the culmination of all the days of penance and an important feast day for women older than 12 and men older than 13. In contrast to all the other Jewish feasts, this day is also celebrated when it falls on a Sabbath, which is actually the day of rest in the Jewish week. Physical joy is allowed on Yom Kippur. From short before sunset the day before Yom Kippur until the next sunset eating and drinking is not allowed. Also any kind of care for the body, like washing and make-up, are prohibited. In addition to all the bans that count on the other Jewish feast, on Yom Kippur all sexual activities are forbidden, too.
In order to make peace with God one has to in get all the relationships with other people straight. Jews have to apologize for all the times they mistreated someone. The day is shaped by five services. They are structured into the Kol Nidrei-prayer on the night before, the Schacharit-prayer in the morning, the Mussaf-prayer at the Holy Temple, which explains the holiday extensively, the Minche-prayer, reading from the book of Jonah and at last the Ne'ila prayer, which is the highlight. The blowing of the shofar ends Yom Kippur officially.
The goal of all that renunciations is to not lay too much importance on the physical needs and the material world but concentrate instead on the prayers in the synagogue, remorse and reconciliation with God. Yom Kippur reminds the Jews of the days back when the golden calf was created on the Mount Sinai and the Jewish became unloyal. When God made peace with the Jews anyway He pronounced this day as the Day of Reconciliation.
Note: Jewish Holidays start at the evening before the very date.