It's called the World Meeting of Families, and for part of 2015, it will make Philadelphia the center of the Catholic world.
"I am very excited that Pope Benedict decided that of all the cities that he could go to, he decided he wanted to be in Philadelphia," Mayor Michael Nutter said on Monday.
The Vatican-sponsored World Meeting of Families are massive several-day-long events held every 3 years.
On Sunday, at this year's event in Milan, Italy, Pope Benedict made this announcement:
"I now have the joy of announcing the next World Meeting of Families will take place in 2015 in Philadelphia in the United States of America," the pontiff said.
It is not yet clear why Philadelphia was chosen.
The gatherings are a celebration of the family unit. In the Church's view, that is the deep and loving union of one man and one woman for mutual support and the nurturing of children.
"It's an occasion for all of us to be renewed, to recover our understanding of family and marriage, Christian family and Christian marriage,.and this will be capped by a possible visit by Holy Father in 2015," said Father Dennis Gill of the Philadelphia Archdiocese.
Father Gill was a young seminarian in 1979 when Philadelphia hosted Pope John Paul the Second. At that time, there was huge outdoor mass.
"Here at Logan Circle was the sanctuary, the alter area, and there were hundreds of thousands of people here," he said.
It is not yet certain where specifically the 2015 gathering will be held.