Yom Kippur, also known as the Day of Atonement, is the holiest day of the year in Judaism. Its central themes are atonement and repentance. Jewish people traditionally observe this holy day with an approximate 25-hour period of fasting and intensive prayer, often spending most of the day in synagogue services.
A controversial aspect is the "Kol Nidre" which includes a renunciation of vows. See the "External links" section.