VATICAN CITY (CNS) — Celebrating the first vespers for Divine Mercy Sunday, Pope Francis formally will deliver the “bull of indiction” or proclamation of the extraordinary Holy Year of Mercy.

Portions of the document will be read in front of the Holy Door at St. Peter’s Basilica April 11, the Vatican announced. The Holy Door, usually bricked up, is opened at the beginning of a jubilee year.

The Holy Year of Mercy is scheduled for Dec. 8, 2015, to Nov. 20, 2016.

The other major basilicas of Rome also have Holy Doors that are opened for jubilee years. The papal document proclaiming the year of mercy will be read April 12 at the Basilica of St. John Lateran by Cardinal Agostino Vallini, vicar for Rome; the Basilica of St. Mary Major by Cardinal Santos Abril Castello, archpriest of the basilica; and the Basilica of St. Paul Outside the Walls by U.S. Cardinal James M. Harvey, archpriest of St. Paul’s.

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.

Announcing the publication of the papal bull, the Vatican press office also explained how formal papal documents came to have that name: “The term bull — from the Latin ‘bulla’ meaning ‘bubble’ or, more generally, a rounded object — originally indicated the metal capsule used to protect the wax seal attached with a cord to a document of particular importance to attest to its authenticity and, as a consequence, its authority.

“Over time, the term began to be used first to indicate the seal, then the document itself, so that nowadays it is used for all papal documents of special importance that bear, or at least traditionally would have borne, the pontiff’s seal,” the statement said.

The papal bull for a holy year, it said, indicates its opening and closing dates and “main ways in which it will be implemented.” It is considered the primary document for understanding “the intentions and the outcomes hoped for by the pontiff.”