This, the holiest day of the Jewish Year is The Day of Atonement. It actually goes from sundown last night to sundown today.
The holiday dates from the time of Moses and the Jews wandering in the desert. After God handed Mose the ten commandments on Mount Sinai, Moses came down to find the people worshipping the golden calf. He got very angry and smashed the stone tablets on the ground. After the people endured a period of atonement for this sin, Moses went back up the mountain and received a second set of tablets from God. When he came back down the people concluded their period of atonement with a special day of atonement. They resolved to do this every year since. Thus, Yom Kippur.
During the times of Ancient Israel, this was the special day that the High Priest could enter the Holy of Holies where the Ark of the Covenant was kept. He performed rituals to ask God for forgiveness for the people.
After the destruction of the Great Temple by the Romans in 70 AD, the day was still observed by the Rabbis in the temples spread throughout the Jew’s Diaspora.
The holiday, observed today, has a long history.