Yom Kippur or the Day of Atonement is celebrated on Tishri 10th. It is the last day of the 40 day period of Teshuvah.
In 2019, it is Wednesday, October 9 on our Gregorian calendar. The holiday officially is 10 days after the civil new year (Yom Teruah/Rosh Hashanah/Feast of Trumpets) is declared at the sighting of the new moon. The new moon was declared on Tuesday, September 29 outside of Jerusalem by at least two witnesses.
This holiday is viewed at the holiest day of Judaism. The day is a Sabbath day of rest. Yom Kippur is the only day a fast is required. The day begins at sun down. On the Day of Atonement, the high priest was kept awake during the dark of the night. The high priest is dressed in a white robe. Sacrifices of a bull, ram, and a goat occur. The priest makes a sacrifice for himself and for the nation of Israel. Yom Kippur is the day the high priest came into the inner sanctuary of the temple to make sacrifices on behalf of the nation of Israel. He is the only person allowed into the inner sanctuary. This day is the only day the high priest is allowed in the inner sanctuary. He entered the highest holy place with incense and coals. The incense is burned. Next, the recovered blood was sprinkled on the mercy seat, the top of the Ark of the Covenant. This is the busiest day of the year for the high priest. See Leviticus 16 for specific details. New Manners and Customs of Bible Times
In the 21st century, we have a high priest who is seated at the right hand of the Father.
Hebrews 6:20 …where Jesus has entered as a forerunner for us, having become a high priest forever according to the order of Melchizedek.
Today animal sacrifices are no longer necessary, even when the Jewish Temple is rebuilt. Jesus is the sacrificial lamb of God.
Hebrews 9:11-14 But when Christ appeared as a high priest of the good things to come, He entered through the greater and more perfect tabernacle, not made with hands, that is to say, not of this creation; and not through the blood of goats and calves, but through His own blood, He entered the holy place once for all, having obtained eternal redemption. For if the blood of goats and bulls and the ashes of a heifer sprinkling those who have been defiled sanctify for the cleansing of the flesh, how much more will the blood of Christ, who through the eternal Spirit offered Himself without blemish to God, cleanse your conscience from dead works to serve the living God?
Hebrews 13:11-12 For the bodies of those animals whose blood is brought into the holy place by the high priest as an offering for sin, are burned outside the camp. Therefore Jesus also, that He might sanctify the people through His own blood, suffered outside the gate.