Factbox-What is a Roman Catholic Holy Year?

VATICAN CITY (Reuters) - Pope Francis started the countdown on Thursday to the 2025 Holy Year, or Jubilee, which is one of the Roman Catholic Church's most important events and is expected to draw tens of millions of pilgrims to Rome.

But what exactly is a Holy Year?

JUBILEE

A Holy Year usually happens once every 25 years, unless a pope calls an extraordinary one to call attention to a particular issue or celebrate a momentous event.

The first Roman Catholic Holy Year is believed to have been instigated by Pope Boniface in 1300.

The last ordinary Holy Year was held in 2000 under Pope John Paul II. The last extraordinary Holy Year was called by Francis in 2016 to emphasise his desire to present the Church as merciful and welcoming rather than moralizing and remote.

During a Jubilee, Catholics can obtain special indulgences, or remission of their sins, if they fulfil certain conditions and do good works or make pilgrimages.

The Vatican website says a Holy Year should also be a time of reconciliation with adversaries and an occasion to promote solidarity, hope and justice in the world.

INDULGENCES

An indulgence is the remission of punishment for a sin that has already been forgiven by a priest. Catholics say that after an indulgence the soul is totally purified of that sin in this life and the next, maybe helping them get to heaven faster.

The Church teaches that people who do not go straight to heaven after death must first spend time in purgatory, a sort of unpleasant waiting room before they can pass through the pearly gates. Indulgences can reduce time in purgatory.

People can get indulgences if they do good deeds like visiting the sick, or make pilgrimages to a "Holy Door" in designated religious sites around the world during a Holy Year.

Centuries ago, people could buy indulgences. Martin Luther was scandalised by this practice, which was one of the controversies that sparked the Reformation in the 16th century.

HOLY DOOR

The Holy Doors, which symbolise the doorway of salvation for Catholics, are only opened during Jubilee years. There are four in Rome -- in St. Peter's Basilica, St. John Lateran, St. Mary Major and St. Paul Outside the Walls.

Their openings will be staggered, but St. Peter's will be the focal point, with its door opening on Dec. 24 and closing on Jan. 6, 2026.

Traditionally, pilgrims passing through these doors secure the remission of their sins. Other Holy Doors will be designated around the world, with Francis saying he wanted to open one in a prison.

(Reporting by Isla Binnie and Crispian Balmer; Editing by Frances Kerry)