Yom Kippur - The Great Day of Atonement
One day each year the High Priest would draw aside the Veil and enter the Holy of Holies to make atonement (Heb. Kafar) and cover the nations sins from the judgement of God and receive forgiveness. It took place on the 10th day of the 7th month, Tishri. By our calendar that would be around the end of September or early October.