Over the past several months, Philadelphia has celebrated — at time agonized over — the arrival of Pope Francis for the World Meeting of Families. 

But here’s a fun fact that you might not know: The World Meeting of Families starts days before Francis’ arrival. 

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Beginning on Tuesday, some 20,000 Catholics from around the globe will take part in a four-day Catholic convention held at the Pennsylvania Convention Center. They’ll take Mass every day, and participate in breakout sessions with titles like “Can Society Exist Without the Family?" and “No Strings Attached? Responding to Hookup Culture.” 

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The church holds a World Meeting of Families every three years. It’s first, held in Rome in 1994, drew just 800 people. It has grown steadily since then. 

In addition to the academic conferences, there is also a youth conference — those attendees will participate in swing dancing exhibitions with nuns, and play Wii with bishops.

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Outside the convention hall, attendees have the opportunity work on a mural. There’s also a special exhibit of biblical artifacts, including fragments of the Dead Sea Scrolls and a first-edition King James Bible. Organizers say volunteers will pack 200,000 meals for people in West Africa during the convention.

All of this leads to the Francis Festival, a big party on the Ben Franklin Parkway Saturday, hosted by entertainer and movie star Mark Wahlberg. Pope Francis is expected to attend that event. Hundreds of thousands of people could attend that event. 

The next day, the pontiff will hold a papal mass on the Ben Franklin Parkway, where crowds are expected to swell to more than 1 million.