Of Roman birth, St. Stephen I governed the Church under the Emperors Valerian and Gallienus. In spite of the most violent persecutions he regularly celebrated the holy Mysteries and held councils in the crypts of the martyrs. He forbade the re-christening of Christians baptized by heretics. In 257 towards the end of the Mass he was saying, he was surprised by the persecutors and beheaded while he sat on his pontifical chair.
Sacerdotes ejus induam salutari, et sancti ejus exsultatione exsultabunt. * Memento, Domine, David: et omnis mansuetudinis ejus.
I will clothe her priests with salvation, and her saints shall rejoice with exceeding great joy. * O Lord, remember David and all his meekness.
(Psalm 131:16, 1 from the introit of Mass)
O God, who givest us joy by the annual solemnity of blessed Stephen Thy martyr and bishop, mercifully grant that we may rejoice in his protection whose birthday we celebrate.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/14288a.htm