Today is Pentecost, or Whitsunday, commemorating the Descent of the Holy Ghost upon the Apostles fifty days after the Resurrection of Christ. The event is held to be the commencement of the temporal Church, as the Apostles were thereby commissioned to their pastoral duties.
The ancient Jewish Pentecost - the Feast of Weeks, or Shavuot - took place 50 days after Passover and was considered second in importance among Jewish feasts. Passover celebrated the redemption of a nation in slavery, whereas Pentecost focused upon God's revelation of purpose (as through the delivery of the law to Moses).
Here in Italy, it's customary to scatter rose leaves from the ceiling of the churches to recall the miracle of the fiery tongues which appeared over the Apostles as they received the gift of the Holy Spirit. Pope Benedict XVI's Pentecost Sunday homily is here and his address surrounding the midday Regina Caeli in St. Peter's Square is here.