History provides very little reliable information about St. Sebastian, but the spread of his cult has withstood the millennia and is still very much alive today. Historical sources, such as the "Depositio martyrum" dating back to 354 and St. Ambrose's "Commentary on Psalm 118" (340-397), give us his dies natalis, his birthday into heaven, as January 20th, 303.
The few reports were later expanded and embellished by the later Passio, written in the fifth century by the monk Arnobius the Younger. St. Sebastian, according to St. Ambrose, was born and raised in Milan to a father from Narbonne (France) and a mother from Milan.  Educated in the Christian faith, he moved to Rome in 270 and embarked on a military career around 283, becoming tribune of the first cohort of the imperial guard in Rome. Emperors Maximian and Diocletian, not suspecting that he was a Christian, esteemed him for his loyalty and intelligence.
Because of his function, he could discreetly help imprisoned Christians, attend to the burial of martyrs, and succeed in converting military and court nobles. According to tradition, he was arrested while burying the martyred saints Claudius, Castorius, Symporian, and Nicostratus, known as "the Four Crowned", on the Via Labicana. He was taken by Maximian to Diocletian, who was already enraged by rumors that Christians were lurking in the imperial palace even among the praetorians.
Diocletian thus apostrophized the tribune: "I have always kept you among the mayors of my palace, and you have operated in the shadows against me, insulting the gods." Sebastian was condemned to martyrdom. He was pierced by arrows shot by the handful of soldiers he commanded in an area of the Palatine Hill called the campus. Believed dead by the soldiers, he was left to be eaten by wild animals. The noble Irene went to retrieve his body to give him burial, but she found that the tribune was not dead and transported him to her home on the Palatine, where she treated his many wounds. 
Miraculously, Sebastian managed to recover; then, despite advice to flee Rome, he decided to proclaim his faith again before Diocletian and his associate Maximian at the temple dedicated to the Sun god. Hearing Sebastian's rebukes for persecuting Christians who were innocent of the charges against them, Diocletian ordered that this time he be scourged to death.
The execution took place in about 304 in the Palatine circus. His body was then thrown into the Cloaca Maxima (the sewers of Rome), so that Christians could not recover it.The martyr appeared in a dream to the matron Lucina, pointing out the place where the corpse was and asking her to bury him in the cemetery ad Catacumbas on the Appian Way.
Until the entire 6th century AD, pilgrims to the Memoria Apostolorum, where Sts. Peter and Paul were honored, visiting also the tomb of the martyr, whose figure had become very popular. In 680 AD, his intercession was attributed to the end of a severe plague epidemic in Rome. St. Sebastian, therefore, was elected thaumaturge against epidemics, and the basilica was named after Sancti Sebastiani. Pope St. Caius called him "Defender of the Church" for his work of healing Christians.
Venerated on January 20th, Sebastian is considered the third patron saint of Rome along with the two Apostles Peter and Paul.