The beginning time of Yom Kippur will differ according to geographic location. To find out what time a Jewish holiday begins in any location, refer to a Jewish calendar or to the Orthodox Union website.
All Jewish holidays begin at sundown because the Jewish people believe that the day begins in the evening and ends the next evening. Yom Kippur begins at sundown on the day before the date specified on the Jewish calendar for the holiday and ends an hour after sunset on the specified date. Fasting and prayer lasts for the entire 25-hour period, and observers often spend much of the time in the synagogue.
Yom Kippur, or the Day of Atonement, is the holiest day of the year for the Jewish people. It's a very culturally significant holiday, and even secular Jews often attend synagogue on Yom Kippur. The holiday is always celebrated on the tenth day of the seventh month, which on the Jewish calendar is Tishri. This month falls between September and October. The dates of Jewish holidays differ every year because the Jewish calendar is not strictly a lunar calendar. The Jewish calendar compensates for changes in the lunar cycle by adding a month when necessary.