"I've been overwhelmed with the positive reaction we've gotten," said DiFrancesco, who launched the group's website, www.massmobphilly.org, in February.
Mass Mob Philly was modeled after a program launched last year in Buffalo, N.Y., DiFrancesco said. He learned about Mass Mob Buffalo while reading an article at a news site and was immediately inspired.
"This struck me as a really good idea," DiFrancesco said. "So I just decided to try to make it happen in Philly and see what happens."
The goal is to bring the faithful from throughout the archdiocese together to get to know each other.
"It's also an opportunity to see some of the great churches Philadelphia has to offer," DiFrancesco said. "Some of them are 100 or more years old and are just beautiful, and people aren't necessarily exposed to them."
St. Francis Xavier, at 24th and Green Streets, was chosen for the first "flash Mass" because he is a member of the parish, DiFrancesco said. Also, the impressive, gray-stone Gothic structure built in 1906 can accommodate 800 worshipers.
The Rev. Philip Bochanski, a priest at St. Francis Xavier, said the parish had embraced the effort.
"We have lots of young people, even in our own neighborhood, who maybe aren't getting up every Sunday to go to church. The question is, how do you reach them?" he asked. "These days, social media tends to be a very effective way to do that."
He said the church has four Masses on Sundays, with about 100 people attending each one. The priest said he was unsure how many people might attend the flash Mass, scheduled for 11:15 a.m. Sunday. The group's Facebook page, linked to its website, has received about 300 "likes."
What's ahead for those who show up? There will be a Mass followed by a reception for everyone.
Christopher Byrd, who organized Mass Mob Buffalo in November, said Mass mobs are being planned in about a dozen cities. The Buffalo program, which is planning to have Masses every other month, has drawn hundreds to two Masses there, one attracting a nearly overflow crowd.
"At the second Mass, people just kept coming in and coming in," Byrd said. "We would have been happy if 50 showed up."
He said the collection plates "had to be passed again and again because they were so full of money."
DiFrancesco said Mass Mob Philly aims to have about four flash Masses this year and that various churches have expressed interest in hosting them.
"I think this is a manifestation of something unique that Catholics and Christians in general can bring to the conversation in terms of modern culture and the use of technology," he said.