Pope St. Sixtus II, mentioned in the Canon of the Mass, was the first martyr actually put to death in the catacombs themselves. Under the Emperor Valerian (258) he was beheaded on his throne just as he finished Mass. His four deacons were put to death with him, and two others, Felicissimus and Agapitus, were decapitated the same day while St. Lawrence, the archdeacon, was put to death three days later.

(Source: The New Roman Missal, Rev. F. X. Lasance)

The festival was originally commemorated on the Vigil of Ember Saturday in Lent. With the decline of liturgical understanding a special festival was instituted in its honor in order to arouse popular devotion to this mystery. It is now the patronal feast of the Lateran Basilica, and of all churches under the title of St. Savior.