This coming Sunday is the Roman feast of St. John in Oil. This is the chapel that they built over the spot where the Romans tried to kill St. John the Evangelist by the Latin Gate. By a miracle, he was saved even though they put him in a caldron of oil (as in the Book of Daniel).
When you go to visit this chapel, first go to the Basilica di San Giovanni a Porta Latina. There ask for the key to the Oratorio di San Giovanni in Oleo e Porta Latina which is the little chapel across the street where the miracle happened. When you're done with your visit just drop the key in the mailbox.