The term "Jubilee" designates a specific year, deriving its name from the ceremonial instrument used to inaugurate it—the yobel or ram's horn, traditionally employed to announce the Day of Atonement (Yom Kippur). While this (Jewish) festival recurs annually, it assumes heightened significance when it heralds the commencement of a Jubilee year. Rooted in biblical teachings, the Jubilee year occurs every 50 years, signifying an "extra" year within the seven-week cycles of seven years, totaling 49 years (see Leviticus 25:8-13). Despite logistical challenges, it was envisioned as a time for restoring a harmonious relationship with God, fostering unity among people, and promoting stewardship of creation. Central to its observance were acts such as debt forgiveness, the restitution of wrongly acquired land, and allowing fields to lie fallow. 

The Gospel of Luke echoes the prophet Isaiah's vision of Jesus' mission, emphasizing the proclamation of liberty, recovery of sight, and the ushering in of an acceptable year to the Lord (Luke 4:18-19; cf. Isaiah 61:1-2). Jesus exemplifies these principles in his daily interactions, fostering liberation and transformative change in relationships. 

In 1300, Pope Boniface VIII initiated the first Jubilee, also termed a "Holy Year," symbolizing a period during which God's holiness brings about transformation. The frequency of Holy Years has evolved; initially observed every 100 years, Pope Clement VI reduced the interval to 50 years in 1343, and in 1470, Pope Paul II further shortened it to 25 years. Additionally, there have been "extraordinary" Holy Years, such as Pope Pius XI commemorating the 1900th anniversary of the Redemption in 1933 and Pope Francis declaring the Year of Mercy in 2015 as an extraordinary jubilee. The observance of Jubilee Years has also evolved over time, initially involving pilgrimages to the Roman Basilicas of St. Peter and St. Paul and later incorporating rituals like the Holy Door. Participation in the Holy Year grants individuals a plenary indulgence. 

https://www.iubilaeum2025.va

Letter of the Holy Father Francis for the Jubilee 2025:
https://www.iubilaeum2025.va/en/giubileo-2025/lettera-di-papa-francesco.html