Yom Kippurthe Day of Atonement, the holiest day of the year. The prohibition against work is similar to that of Shabbos, and additionally the day is spent fasting, even when Yom Kippur falls on Shabbos. It is the only day of the year in which there are five prayer services, and the day is particularly asociated with the forgiveness of sin. In Temple times it was distinguished by the unique service of the High Priest in the Holy of Holies.
Source: Binyomin Kaplan.