While many Jewish holidays were established by rabbinic tradition, Yom Kippur has been a sacred day for Jewish people since the ancient Israelites journeyed through the desert. The holiday is referenced multiple times in the Bible: twice in Leviticus, and then again in the Book of Numbers.
In Leviticus 16:29, the God tells the Israelites that the tenth day of the seventh month (Tishrei, according to the modern Jewish calendar) should be a day on which they afflict themselves, abstain from working and be cleansed of their sins.
The 16th chapter of Leviticus offers a detailed description of the various rituals performed by the High Priest in the ancient Temple on Yom Kippur, the one day of the year he was permitted to enter the Temple’s innermost sanctum, the Holy of Holies.The priest would sprinkle the blood of sacrificial animals on the ark and symbolically cast off the nation’s sins by releasing a goat into the desert.
In the 23rd chapter of Leviticus, the commandment to observe Yom Kippur is repeated, with the addition that anyone who performs work or fails to fast on Yom Kippur will be cut off from the people.
The commandment to observe Yom Kippur is reiterated again in the 29th chapter of Numbers, which also specifies the types of sacrifices to be brought on that day.
Since the destruction of the Temple, Jews have ceased to observe Yom Kippur as it was done in ancient times. However, the passages from Leviticus 16 are still read in many synagogues on Yom Kippur it, and many Yom Kippur prayers excerpt the passage from Numbers.
Source: Jewish Living