The Missal gives to-day the Octave of St. John the Baptist, which, however, only appears in the Roman Calendar during the late Middle Ages. In the liturgical reform of Pius X this day was chosen for the feast of the Most Precious Blood, which had already been fixed under Pius IX for the first Sunday of July. Pius IX instituted this feast in thanksgiving for the deliverance of the Apostolic See from the violent revolutionaries who had expelled the Pope to Gaeta. In 1849, with the assistance of the French army, they were vanquished and the pope was able to return to Rome.
The meaning of this festival is closely akin to that of the Sacred Heart. The Precious Blood is the price of universal redemption, which love would not have to be anything less than itself. There is a very close connection between the Heart and the Blood not only because, according to St John, after the death of Jesus, blood and water flowed from His wounded Heart, but because the first chalice in which that divine Blood was consecrated and vivified was precisely the Heart of the incarnate Word.
The apostle of this special devotion was the Blessed Gaspare del Bufalo, founder of the congregation of the Most Precious Blood.
The Mass is of quite recent composition. In the ancient Roman rite the Mass of Passion Sunday was especially intended to recall to the remembrance of the faithful the infinite value of the Blood of Jesus Christ.
(Source: The New Roman Missal, Rev. F. X. Lasance)