It is then and there that they may make the first of many consequential decisions - when to start the conclave to elect the next pope.
Despite the rest of the world's desire for these men to reach a decision, they are in no rush to do it.
"The vacancy stated Thursday, they're not meeting until Monday. They're taking their good time," said Rocco Palma, Catholic Church commentator.
The cardinals may prefer that all 115 be present for a date to be set but, as of Sunday night, it's not certain that will be the case in time for the meeting.
In a city rich with tradition and centuries of protocol, recent events have created a lot of uncertainty.
"Because of the circumstances of resignation, it's been a completely different world," said Palma.
Ordinarily, a conclave would follow a period of mourning after the last pope's death.
The last time a vacancy came because of a resignation, back in 1294, this Saint Peter's Basilica wasn't even opened yet. It came more than 300 years later, in 1604.
The rarity of the moment was not lost on the faithful, who packed Saint Peter's Square.
"It's a momentous occasion," said Collette King.
As they wound their way into the Basilica for Sunday morning mass, there was a somewhat unusual sentiment.
Collette and her sister Eileen King came here from England, two of the tens of thousands who came to see this historic place, and will now watch history unfold.
"We're so excited to know who we'll have next, what kind of pope it'll be, and what will happen to the Catholic Church," said Eileen.
It's a similar sentiment on the streets outside of Vatican City.
However along with the excitement for a new pope, there's certain sadness over the one that just resigned.
Signs dot the streetscape in the city, signs that depict his photo with the words 'You will be with us always, thank you.'
For now many questions still remain regarding who the new pope could be but all in all many Catholics are just hoping for a smooth transistion.
