The free, online registration will not be activated until October, but in late August the Vatican published a brief explanation of the plan on www.im.va, which is the website for the Year of Mercy that begins Dec. 8.

"Since a large number of pilgrims is anticipated, it is strongly recommended that pilgrims register their arrival in Rome and the possible times at which they wish to make their pilgrimage," the notice said. People with special needs, particularly regarding mobility, can enter that information on the reservation form and be assured of assistance when they arrive.

Knowing how many pilgrims to expect on a given day and giving them an appointment so they can avoid hours of waiting in line will "ensure that the pilgrimage remains prayerful," the Vatican said. The Archdiocese of Turin used a similar system to regulate the pilgrim flow April 19-June 24 for the public exposition of the Shroud of Turin.

Not reserving a date and time in advance will not exclude a pilgrim from crossing the Holy Door's threshold, the Vatican said; a special pilgrims' office will be open along the main boulevard leading to St. Peter's Square and pilgrims can make last-minute reservations there.

"The pilgrimage to the Holy Doors at the other papal basilicas of Rome (St. John Lateran, St. Mary Major and St. Paul Outside the Walls) will not be subject to registration," the Vatican said.

The Holy Door at St. Peter's Basilica is located to the right of the basilica's main entrance; between Holy Years it remains sealed with bricks as a symbolic reminder of the barrier of sin between human beings and God.

At the beginning of a Holy Year, the pope opens the door to signal the opening of a special year of grace and pardon.