The 10th day of the 7th Month marked the Day of Atonement, however, every 50th year, it also marked an event that was integral to the economic system of Ancient Israel--the Jubilee. Inside of each seven year block, there was a cycle of tithe years and land sabbaths, which after seven of these cycles culminated in the Year of Jubilee. During this year, debts were forgiven, people were able to take possession of their family lands which may have been sold, and the economic system was reset. It was intended to prevent oppression, indebtedness, and poverty.
Why was it announced on the Day of Atonement?
Because it parallels what Jesus Christ did for us. His sacrifice on our behalf, His blood poured out for us enables us to be reconciled to God and releases us from our debt, His sacrifice releases us from bondage to sin, and restores to us our inheritance as children of God.