So did a little quick research. The tradition of rose petals through the oculus is over 1,400 years old. Started in the time of the Roman Empire. . The Pantheon was given by the Roman Emperor to Pope Boniface IV and has been a Catholic church since around 600 AD and the tradition has continued. As Mass ends every Pentecost the choir chants the 'Veni Sancte Spiritus' and the rose petals are release onto the crowd below to symbolize the Holy Spirit descending upon the apostles (and parishioners in the church) in the form of fire. Nice tradition.