Chaput was in Milan, where the Pope made the announcement over the weekend. As it turns out, Chaput found out a month ago but managed to keep the secret, telling just a few trusted advisors.
Chaput said he has no idea why Philadelphia was chosen.
"People have asked me that, about the motivation of the pope for choosing Philadelphia and any answer I give would be simply guessing," he said. "Your guess is as good as mine."
The Archbishop says he was reluctant at first considering the Archdiocese's desperate financial condition. But, he says, they'll get help from private donors.
"It's going to cost us something up front but, hopefully, not very much. Hopefully we'll get into the fundraising soon and that will cover the costs," said Chaput.
In 1979, a million people crowded the Parkway to see Pope John Paul II. Archbishop Chaput says he wouldn't be surprised if a similar crowd came out to see Pope Benedict XVI.
The conference is expected to last about five days with the pope coming in for the closing benediction.
It's expected to be a financial boost for the city and a spiritual boost for Catholics in the Archdiocese. Over the past few years, they've been battered with bad news from the sex scandal to school and parish closings.
"This was certainly was an unexpected gift, and I know it's from God, and it will be a great blessing for us," Chaput said.
Pope Benedict will be 88 years old in 2015. However, Archbishop Chaput says the pope appears to be in great physical shape and he fully expects him to be here.