The feast of Pentecost recalls the event detailed in the Acts of the Apostles, occurring 50 days after Our Lord's Resurrection:
When the time for Pentecost was fulfilled, they were all in one place together. And suddenly there came from the sky a noise like a strong driving wind, and it filled the entire house in which they were. Then there appeared to them tongues as of fire, which parted and came to rest on each one of them. And they were all filled with the Holy Spirit and began to speak in different tongues, as the Spirit enabled them to proclaim.
The feast is often referred to as the "Birthday of the Church", and focuses especially on the coming of the --Holy Spirit, as promised by Christ. The symbolism of the day is the color red--originating from the description of the tongues of flame. Say what you will about the Romans, but they love a good spectacle. So, this event of Pentecost is given firmer form here in Rome when thousands of red rose petals are dropped down from the hole of the Pantheon (fittingly enough, by Roman firemen, I'm told) after the main Mass of the day.
I was not able to attend this year, but I did see it last year. Here are my pictures form Pentecost 2012 at the Pantheon: