A look inside the Cathedral Basilica of Saints Peter and Paul

"When you walk into the cathedral, there's a confluence of images that takes the person in right away... the paintings and mosaics, lovely pieces of furniture, the great organ, the sanctuary, lateral altars - it all comes together as one."

But for Father Dennis Gill - Director of the Office of Divine Worship - nothing is more glorious than the main altar.

This is where Pope Francis will celebrate mass - it was added a century after the basilica was built.

"The apse or the round part of the sanctuary that we see was added in the mid-50's."

Adding a different - more old world European style with a stunning free-standing altar -- the word of God or ambo in gold and the cathedral or chair from which the bishop presides as chief priest.

"I wouldn't say it's ornate I would say it's more restrained believe it or not. It could be much more ornate."

More than 9 chapels or shrines surrounding the sanctuary - some of these are altars from churches and parishes that were closed. Treasures like the stations of the cross - now preserved here.

"The cathedral was built at a time when there was an anti-Catholic sentiment in the city - there wasn't a lot of money, but they did a magnificent job."

A full time maintenance team - led by Nick Martino - keeps up this massive old structure.

"We want the place to shine, really sparkle in every way."

The Archdiocese would not disclose how much it costs to maintain on a regular basis -- right now -- they're just making sure they're ready to welcome the world into their home.

"In our chapel next door we will have 24 hour a day seven days a week adoration of the most blessed sacrament - so the chapel will not close."

More priests on hand at/for confessionals, more guided tours - but you'll need to arrange to trip to the crypt below the altar.

The remains of all but a few bishops and archbishops of Philadelphia rest in the walls and vaults.

Mass is regularly celebrated here for the departed.

There's plenty to see brand new installed on the walls and halls.

The 2015 World Meeting of Families painting - depicting the holy family set inside the altar here.

"This will remain in fact when Pope Francis is here to offer mass we're going to move it closer to the alter then we'll move it back here."

"It endures and it's part of the legacy of Philadelphia and I'm just thrilled that so many people will somehow be able to be here on that day."

A day in history.

"People will be pre-seated here in the Basilica by about 8:30 - the Holy Father won't arrive until about 10:30."

A mass honoring the Blessed Mother - led by the Pope.

"We expect the mass to take about an hour and a half or so. The principal language will be Latin but the biblical text will be in Spanish and English, the music will be in Latin, English, Spanish, and Vietnamese."

It's for the local clergy - bishops, priests, deacons and others of the church and people who already have passes to attend.

"We're going to full the church until we can't fit any more people."

Father Gill hopes the spirit to be felt well past the pope departure

"I do say all the time that everyone is welcome and it's true. I want it to be the people's home, for many different reasons, in Center City Philadelphia."