Vatican spokesman the Rev. Federico Lombardi told reporters today that Benedict will be in St Peter's Square for the canonisation of John and John Paul. He said Benedict and many cardinals will "concelebrate" the Mass with Francis.
Benedict resigned from the papacy a year ago, and since has largely dedicated himself to prayer in a monastery on the Vatican grounds. His appearance tomorrow will be his highest-profile one since he retired.
"We will all be happy to have his presence" at the ceremony, the Vatican spokesman said.
Benedict also showed up Francis' ceremony to elevate churchmen to cardinal's rank in February. But that ceremony was not a Mass, meaning tomorrow's appearance by two popes would be the first Mass concelebrated by two pontiffs, one reigning and the other retired.
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As German Cardinal Joseph Ratzinger, Benedict presided over John Paul II's funeral in the square in 2005. He was soon elected pontiff himself, going on to lead the ceremony to beatify his Polish-born predecessor in 2011, also in the square.
Pilgrims poured into Rome today in big groups or as individual families or travellers, eager to be among those taking their place in the square before dawn on the day of the ceremony.
Interior Minister Angelino Alfano said an estimated 1 million people were expected to flood into Rome for the event. Working to ensure the crowds are safe, authorities were deploying about 3,500 extra police officers in Rome, beefing up the total to around 10,000 officers.