The Feast of Saints Peter and Paul, or the Solemnity of Saints Peter and Paul, is a liturgical feast in honour of the martyrdom in Rome of the apostles Saint Peter and Saint Paul, which is observed on 29 June. The celebration is of ancient origin, the date selected being the anniversary either of their death or of the translation of their relics.
Catholicculture.org has more on the feast day HERE.
"Veneration of the two great Apostles, Peter and Paul, has its roots in the very foundations of the Church. They are the solid rock on which the Church is built. They are at the origin of her faith and will forever remain her protectors and her guides. To them Rome owes her true greatness, for it was under God's providential guidance that they were led to make the capital of the Empire, sanctified by their martyrdom, the center of the Christian world whence should radiate the preaching of the Gospel.
St. Peter suffered martyrdom under Nero, in A.D. 66 or 67. He was buried on the hill of the Vatican where recent excavations have revealed his tomb on the very site of the basilica of St. Peter's. St. Paul was beheaded in the via Ostia on the spot where now stands the basilica bearing his name. Down the centuries Christian people in their thousands have gone on pilgrimage to the tombs of these Apostles. In the second and third centuries the Roman Church already stood pre-eminent by reason of her apostolicity, the infallible truth of her teaching and her two great figures, Sts. Peter and Paul."
Phil over at Blue Eyed Ennis has a great post about the feast day with links, videos and suggested readings about these pillars of the church.
In Rome, the Solemnity is also the day for the imposition of the pallium on the newly appointed archbishops. The pallium is a white scarf made of wool that carries six black crosses, it's meant to symbolize each bishop's unity with the pope. At the beginning of the Mass each pallium is laid over the tomb of St. Peter and then handed to the pope before being presented. The full list of archbishops receiving the pallium this year is available here.
This year as well, there is an ecumenical theme to this celebration associated with the See of Rome. While it is traditional for representatives of the other christian churches to attend the patronal feast of the See of Rome particularily the representatives of the Ecumenical Patriarch of Constantinople, this year following on from the visit of Pope Benedict XVI to England, the world famous Westminster Abbey choir is here in the Vatican to sing alongside the Sistine Chapel choir at the Papal Mass for the feast of Saints Peter and Paul on Friday morning. In issuing the invitation, Pope Benedict stressed that such an event may serve to encourage the enriching mutual exchange of gifts between the two liturgical and cultural traditions. Further reports about the visit of the choir are available from Vatican radio HERE and HERE.
Also, for the current successor of St Peter, it is a special feast day as it is the anniversary of the ordination to the priesthood of Joseph Ratzinger, 61 years ago this year. Ad multos annos!