All else (his youth, his martyrdom by arrows) is fiction, some of it dating from more than a thousand years after his death. But what we know is what we need to know. For the Christians of the fourth century the important, the true, the sufficient fact about Sebastian was that he was a martyr, and they venerated him as such. It should be enough for us as well. See the article in the Catholic Encyclopaedia.

  ‘Saturdays stand out among those days dedicated to the Virgin Mary. These are designated as memorials of the Blessed Virgin Mary. This memorial derives from Carolingian times (9th century), but the reasons for having chosen Saturday for its observance are unknown. While many explanations of this choice have been advanced, none is completely satisfactory from the point of view of the history of popular piety.

  ‘Whatever its historical origins may be, today the memorial rightly emphasizes certain values to which contemporary spirituality is more sensitive. It is a remembrance of the maternal example and discipleship of the Blessed Virgin Mary who, strengthened by faith and hope, on that “great Saturday” on which Our Lord lay in the tomb, was the only one of the disciples to hold vigil in expectation of the Lord’s resurrection. It is a prelude and introduction to the celebration of Sunday, the weekly memorial of the Resurrection of Christ. It is a sign that the Virgin Mary is continuously present and operative in the life of the Church.’

  Directory on Popular Piety and the Liturgy (2001), §188

  Whenever we take up a Bible we touch Irenaeus’s work, for he played a decisive role in fixing the canon of the New Testament. It is easy for people nowadays to think of Scripture – and the New Testament in particular – as the basis of the Church, but harder to remember that it was the Church itself that had to agree, early on, about what was scriptural and what was not. Before Irenaeus, there was vague general agreement on what scripture was, but a system based on this kind of common consent was too weak. As dissensions and heresies arose, reference to scripture was the obvious way of trying to settle what the truth really was, but in the absence of an agreed canon of scripture it was all too easy to attack one’s opponent’s arguments by saying that his texts were corrupt or unscriptural; and easy, too, to do a little fine-tuning of texts on one’s own behalf. Irenaeus not only established a canon which is almost identical to our present one, but also gave reasoned arguments for each inclusion and exclusion.

  Irenaeus also wrote a major work, Against the Heresies, which in the course of denying what the Christian faith is not, effectively asserts what it is. The majority of this work was lost for many centuries and only rediscovered in a monastery on Mount Athos in 1842. Many passages from it are used in the Office of Readings.