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Pope Francis has announced extraordinary Jubilee year. Reuters

Pope Francis marked the second anniversary of his election announcing a special Jubilee Year to start in December.

The move comes as a surprise as there have been only 26 Holy Years in the history of the Roman Catholic Church.

Francis made the announcement while celebrating a penitential liturgy during a late Friday mass at Vatican's St. Peter's Basilica.

He said the extraordinary Jubilee Year will start on 8 December and help the Church focus on its main priorities: forgiveness and mercy. It will end on 20 November 2016.

The starting date is of particular significance as it marks both celebrations for the Feast of the Immaculate Conception and the 50th anniversary of the closing of the Second Vatican Council.

"Dear brothers and sisters, I have often thought about how the Church could make its mission of being a witness of mercy clearer," Francis said. "It is a journey that begins with spiritual conversion, thus I have decided to call an extraordinary Jubilee focused on God's mercy."

Jubilee years are important to the faithful as they allow them to receive special indulgences, the highest form of forgiveness.

In the past worshippers travelled from all over the world to participate in one of a limited number of events where indulgences were granted.

Indulgences grant a full pardon from the temporal punishment for sins that have already been forgiven in confession. That translates as a shorter penance for the faithful to atone or a shorter time his/her soul will have to spend in purgatory.

The first Jubilee was called with such purpose by Pope Boniface VIII in 1300. Holy Years were initially to be held once every century, but the term was later shortened to 25 years.

Jubilees falling on those years are known as "ordinary" as opposed to the "extraordinary" ones that are called by a pontiff for some special occasion.

The last ordinary Jubilee, known as the "Great Jubilee" was held under St. John Paul II in 2000 to mark the start of the third millennium, while the last pope to call an extraordinary Holy Year was Pius XI in 1933.

Announcing the new Jubilee, Francis said his intention was to keep the church's doors open not to exclude anyone from God's mercy.

"The bigger the sin, the greater must be the love that the church shows to those who convert," he said.