It's official. Pope Francis is coming to Philly.

The pontiff confirmed Monday he plans to attend the highly-anticipated World Meeting of Families in Philadelphia next year.

"I wish to confirm according to the wishes of the Lord, that in September of 2015, I will go to Philadelphia for the World Meeting of Families," he told a gathering of religious leaders in Rome.

The three-day visit will include a public Sunday mass on the Ben Franklin Parkway on Sept. 27, 2015. Officials estimate up to 2 million people could come to see the pope.

"This will be the largest event in the city's modern history," Philadelphia Mayor Michael Nutter said. The mayor called the visit a "massive undertaking" that will require a "tremendous amount of coordination."

The Holy Father made the announcement in Rome during his opening remarks at the Humanum Colloquium. The Humanum Colloquium is a gathering of leaders and scholars, including Chaput, of various religions around the world focused on marriage and family life.

"We look forward to Pope Francis’ arrival in Philadelphia next September and we will welcome him joyfully with open arms and prayerful hearts," said Archbishop Charles Chaput, the leader of Philadelphia's Archdiocese, in a prepared statement.

Chaput said he "applauded the loudest" when Francis made the announcement, according to The Associated Press.

It will be the Pope’s first visit to the United States since he was elected to lead the Catholic church in March 2013. The last time a pope visited Philadelphia was in October 1979 when John Paul II to deliver mass along the Parkway. Over history, only four popes have visited the United States a total of nine times.

The World Meeting of Families will be held from Sept. 22 to Sept. 27, 2015 in Philadelphia. Held every three years, the conference is a celebration of families, love and life, organizers say. It’s also the world’s largest Catholic gathering of families.

Archdiocese spokeswoman Donna Farrell said that Francis is expected to arrive Sept. 25. Besides the Parkway mass, the Pontiff is also expected to attend the intercultural Festival of Families on Sept. 26, according to the archdiocese. The Vatican will reveal detailed plans in spring or summer of 2015.

“As I’ve said many times before, I believe that the presence of the Holy Father will bring all of us — Catholic and non-Catholic alike — together in tremendously powerful, unifying and healing ways. We look forward to Pope Francis’ arrival in Philadelphia next September and we will welcome him joyfully with open arms and prayerful hearts," Chaput said.

Pope Francis' visit has been expected, but was not officially confirmed until Monday's announcement.

In August, NBC News correspondent Anne Thompson asked the Pope, who turns 78 next month, in Italian if he would attend the meeting. Thompson said the Pope told her “yes,” and mentioned “World Family Day.”

Earlier this year, a group of city boosters — including Nutter and Gov. Tom Corbett and First Lady Susan Corbett — went to Rome and a contingency from the Vatican visited Philadelphia.

Recalling the Vatican visit in March, Susan Corbett said the pontiff promised her husband that he would come.

"It was there that the governor had the honor of inviting the pope to Philadelphia and the Holy Father answered our invitation by whispering three words in Tom's ear: 'I will come," she said.

Francis' predecessor, Pope Benedict XVI, had earlier committed to coming to the meeting. But still, the Vatican said the official decision would not be made until a few months before.

The World Meeting of Families has a budget of $45 million, which is being offset by donations from private donors and corporations. Organizers say they're just over halfway to their goal. It's unclear, however, whether the pope's visit will increase the event's total cost.

Registration for the WMOF officially opened Nov. 10. Organizers of the conference expect some 20,000 people to attend its Adult and Child Congresses at the Pennsylvania Convention Center.

Farrell said tens of thousands of additional people are expected to be in the city during the week. Then the number of visitors is expected to grow each day as the Sunday mass nears.

"By the time the weekend arrives for the Festival of Families, I would expect that we would be close to, if not over a million [people] and then on Saturday, well over a million," she said.

The addition of nearly 2 million people to the city's 1.5 million population could temporarily make Philadelphia the third-largest city in the U.S. -- surpassing Chicago and Houston.

Lodging all of those people will be a challenge.

There are 10,500 hotel rooms in Center City and 30,000 hotel rooms overall in the region, according to the Philadelphia Convention and Visitors Bureau.

Farrell said organizers are relying on locals to open their homes to visitors through a "Homestay" program. As part of the program, visitors would pay homeowners a small fee to stay.

Answering questions about security, Nutter said the city will be working with state police, the U.S. Secret Service, FBI as well as Vatican police, Swiss Guard and Italian government police.

"We host big events well here in Philadelphia," Nutter said. "This is slightly bigger, with a few more zeros at the end in terms of the number of folks coming, but we'll be ready."

You can find more information on the upcoming event on the official website. You can also register for the event here.