The Cycle makes us honour to-day a saint whom "God anointed with His holy oil " (Gradual) and whom He invested with the fulness of His priesthood (Introit, Alleluia) by raising him to the pontifical throne after St. Hyginus in 142, others say in 167.
He prescribed that the feast of the Resurrection should only be kept on a Sunday, which thenceforth became the chief of all Sundays.
He established a baptistry in the house which St. Pudentiana and St. Praxedes had placed at his disposal, and where their father, the Senator Pudens, had already received St. Peter. He transformed into a title-church the adjoining baths of Novatus, where is held the Station on the Tuesday in the third week of Lent. On account of the stay of the first Sovereign Pontiff, he dedicated it under the title of Pastor.
To fulfil his office of good shepherd, he feared not to renounce his own life (Gospel), and endured many hardships, which hastened his end, for his sheep and for Christ the supreme Pastor. He received at the same time as the crown of martyrdom the crown of life that God has promised to those who love Him (Epistle), and was buried in 150 on the Vatican.
Statuit ei Dominus testamentum pacis, et principem fecit eum: ut sit illi sacerdotii dignitas in aeternum. * Memento, Domine, David, et omnis mansuetudinis ejus.
The Lord made to him a covenant of peace, and made him a prince: that the dignity of the priesthood should be to him for ever. * O Lord, remember David: and all his meekness.
(Ecclesiasticus 45:30 and Psalm 131:1 from the Introit of Mass)
Be mindful of our weakness, O almighty God, and since the burden of our deeds is grievous to us, grant that the glorious intercession of blessed Pius Thy martyr and bishop may protect us.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/12126b.htm