In 1854, Pope Pius IX proclaimed Immaculate Conception making Mary free of Original Sin.
Pope Pius IX stopped any doubts about the holiness of Mary. He proclaimed Immaculate Conception for the mother of Jesus as fact. This freed Mary from the Original Sin at the moment of her conception. Original Sin meaning the sin that all men are born into because of the Fall of Adam.
To state simply, the Pope declared Mary sinless from the conception of her birth, making her a holy vessel for the Lord. While most Catholics already considered Mary holy, this announcement made it official. Catholics in the United States celebrate this on December 8th.