Yom Kippur is probably the most important holiday of the Jewish year. Many Jews whos do not observe any other Jewish custom will refrain from work, fast and/or attend synagogue services on this day.
The name "Yom Kippur" means "Day of Atonement," and that pretty much explains what the holiday is. It is a day set aside to "afflict the soul," to atone for the sins of the past year.
Yom Kippur (Hebrew: יוֹם כִּפּוּר or יום הכיפורים), Also known as Day of Atonement, is the holiest day of the year for the Jews. Its central themes are atonement and repentance.
Jews traditionally observe this holy day with a 25-hour period of fasting and intensive prayer, often spending most of the day in synagogue services.
Yom Kippur completes the annual period known in Judaism as the High Holy Days (or sometimes "the Days of Awe").