After months of anticipation the jubilee will get underway with Francis overseeing the opening of the Holy Door, an entrance in St. Peter’s Basilica that is sealed apart from during holy years. The moment will be live-streamed around the world and the event is expected to attract huge crowds to the Vatican, where strict security measures will be in place.

For the first time during a jubilee, holy doors will also be opened around the world, as most of the world’s 1.2 billion Catholics will be unable to make the pilgrimage to Rome.

Francis has declared mercy as the theme of this Holy Year, which runs until Nov. 20, 2016, and has promised to carry out an act reflecting this theme once a month. The first such moment will come on Dec. 18, when the Pope will open a holy door at a hostel run by the Catholic charity Caritas.

The first day of the Holy Year will end with an evening projection of photographs onto the facade and cupola of St. Peter’s Basilica, depicting mercy, humanity, the natural world and climate change.

The Vatican calendar over the following months will be punctuated with events highlighting the needs of particular groups of people, such as prisoners and the disabled. More broadly, a holy year is intended to promote reconciliation and the forgiveness of sins; it occurs every 25 years unless — as in this instance — a pope calls an “extraordinary” jubilee.

In September Francis called on priests to forgive women who repented during the jubilee after having an abortion; the move will not change Church doctrine but reflects the pontiff’s aim to create a more welcoming Church.

Yet actually entering the heart of the Vatican will itself be more difficult during the jubilee, as metal detectors have been installed under the colonnades of St. Peter’s Square. Soldiers have been stationed at Rome’s subway stops since terrorist attacks in Paris on Nov. 13, while a no-fly zone has been put in place that includes banning drones from the skies above the Eternal City.

Speaking ahead of the jubilee’s opening, Italian Archbishop Rino Fisichella, president of the Pontifical Council for Promoting New Evangelization, said the Vatican had worked alongside Rome authorities on security.

“We have every reason to think that there’s a total guarantee regarding the pilgrims who come to Rome, but it’s obvious that it’s necessary to be vigilant, as in every other part of the world,” he said.

The Pope made the surprise announcement of the Holy Year in March, leaving little time for authorities to prepare for the arrival of millions of pilgrims.