“Jubilee” is the name given to a particular year; the name comes from the instrument used to mark its launch. In this case, the instrument in question is the yobel, the ram's horn, used to proclaim the Day of Atonement (Yom Kippur). This (Jewish) holiday occurs every year, but it takes on special significance when it marks the beginning of a Jubilee year. We can find an early indication of it in the Bible: a Jubilee year was to be marked every 50 years, since this would be an “extra” year, one which would happen every seven weeks of seven years, i.e., every 49 years (cf. Leviticus 25:8-13). Even though it wasn’t easy to organise, it was intended to be marked as a time to re-establish a proper relationship with God, with one another, and with all of creation, and involved the forgiveness of debts, the return of misappropriated land, and a fallow period for the fields.
Quoting the prophet Isaiah, the Gospel of Luke describes Jesus’ mission in this way: “The Spirit of the Lord is upon me, because he has anointed me to bring good news to the poor. He has sent me to proclaim liberty to captives and recovery of sight to the blind, to let the oppressed go free, and to proclaim a year acceptable to the Lord,” (Luke 4:18-19; cf. Isaiah 61:1-2). Jesus lives out these words in his daily life, in his encounters with others and in his relationships, all of which bring about liberation and conversion.
In 1300, Pope Boniface VIII called the first Jubilee, also known as a
“Holy Year,” since it is a time in which God's holiness transforms us. The frequency of Holy Years has changed over time: at first, they were celebrated every 100 years; later, in 1343 Pope Clement VI reduced the gap between Jubilees to every 50 years, and in 1470 Pope Paul II made it every 25 years. There have also been “extraordinary” Holy Years: for example, in 1933 Pope Pius XI chose to commemorate the 1900th anniversary of the Redemption, and in 2015 Pope Francis proclaimed the Year of Mercy as an extraordinary jubilee. The way in which Jubilee Years are marked has also changed through the centuries: originally the Holy Year consisted of a pilgrimage to the Roman Basilicas of St. Peter and St. Paul, later other signs were added, such as the Holy Door. By participating in the Holy Year, one is granted a plenary indulgence.