This Sunday, we will hear from Leviticus 16-17, looking at the Day of Atonement in ancient Israel.  The Day of Atonement was an annual fast, when the entire nation would effectively cleanse and re-establish the sacrificial system for the coming year.  The High Priest would enter the Most Holy Place, and sprinkle the mercy seat with the blood of the sin offerings (symbolizing the satisfaction of divine wrath, because of the substitutionary sacrifice).  Then, he would confess the sins of the people onto a scapegoat, and send it out into the wilderness (symbolizing cleansing, as our sin is taken away).

Resources
How To Prepare For Sunday