During a brief ceremony following the 12:10 p.m. Mass June 11 at Holy Name Cathedral, Archbishop Cupich blessed and sealed one of the cathedral's main entryways, which will be used as the official archdiocesan holy door for the jubilee Year of Mercy. The door -- located on the northeast side of cathedral entrance -- will remain sealed, with no one passing through it, until the archbishop reopens it on Dec. 13. The Year of Mercy officially begins Dec. 8 when Pope Francis will open the holy door at St. Peter's Basilica at the Vatican. He has asked that churches and basilicas around the world open their holy doors Dec. 13. Above the outside of the holy door at Holy Name Cathedral hangs the papal seal of a gold and silver key crossed with a tiara above them. The tradition of holy doors for holy years in the Catholic Church began with Pope Martin V in 1423. During a Lenten penance service in March, Pope Francis announced his intention to proclaim the holy year as a way for the church to "make more evident its mission to be a witness of mercy." Holy Years usually are held every 25 years; the last was the great jubilee of the year 2000. Holy years feature special celebrations and pilgrimages, strong calls for conversion and repentance, and the offer of special opportunities to experience God's grace through the sacraments, especially confession. Extraordinary holy years, like the Holy Year of Mercy, are less frequent, but offer the same opportunities for spiritual growth. In addition to an "ordinary" holy year set at 25-year intervals, occasionally a special jubilee is proclaimed to mark some outstanding event. The custom of these so-called "extraordinary" jubilees began in the 16th century, and they can vary in length from a few days to a year. There have been 26 "ordinary" holy years so far, the last one being the Great Jubilee of the Year 2000. There were no jubilee celebrations in 1800 and 1850 because of political turmoil at the time. There have been two extraordinary jubilees in the last century: 1933, proclaimed by Pope Pius XI to mark the 1,900th anniversary of Christ's redemption, and 1983, proclaimed by St. John Paul II to mark 1,950 years since the redemption. Catholic News Service and the Office for Divine Worship contributed to this story.