We don't know a lot about St. Dismas other than what's in the Gospel. We have the tradition of his name, which may or may not be correct. Here is some limited information I was able to bring up about him.
"Our Lord's words on the Cross promising him Paradise have entitled the Good Thief to be registered among the Saints honoured by the Catholic Church. Apocryphal Gospels and other ancient writings assign to him the name of DISMAS, and give various details concerning him. But we have nothing in any way historical to allege. His Feast, though kept on various days, is put in the Roman Martyrology, as by the Greeks, on March 25, from an old belief that Our Lord's Crucifixion, and therefore the Good Thief's confession, fell on that day in the year of the Passion."
(1921). In The Book of Saints (page 126). London: A & C Black.
"The name, from the Greek. δυσμή, 'dying', is found in the 'Gospel of *Nicodemus', chapter 10; cf. C. *Tischendorf, Evangelia Apocrypha (2nd edition., 1876), pages 192 f., 362. A. Bessières, Le Bon Larron, Saint Dismas . H. Lesêtre in Dict. Bibl. 4 (1908), cols. 94–6, s.v. 'Larron'; H. *Leclercq, OSB, in DACL 8 (pt. 1; 1928), cols. 1402–4, s.v. 'Larrons (Les Deux)'."
Cross, F. L., & Livingstone, E. A. (Eds.). (2005). In The Oxford dictionary of the Christian Church (3rd edition revision, page 492). Oxford; New York: Oxford University Press.
"Mentioned once in the gospels (Luke 23:39–43, par.) and several times in the apocrypha: in the Arab Gospel (30), the Gospel of Nicodemus (10), the Visio Theophili and esp. in the History of Joseph of Arimathea.
These works give him various names: Titus, Demas, Dismas. He is mentioned often by the church fathers (Gregory of Nazianzus, John Chrysostom, Jacob of Sarug and others). The books of the apocrypha seek to resolve the question of how one thief was saved and the other not, and recount the story of both: Dismas was educated by Mary and, as a thief, took from the rich and gave to others, whereas the other (Dumachus) had always been evil. Both met the Holy Family in Egypt, the good thief expressing his piety toward them. In the History of Joseph of Arimathea the author dedicates a sizable part to the good thief (1; 3–5), who after his death appears, full of light, to Joseph and John. The cult of the good thief grew in the Middle Ages; he is in the calendar (23 March) and mentioned in the Martyrology of Usuard; various foundations were established under his name to help repentant prisoners."
CANT 78; BS 3, 595–600; AASS Martii III, 543 (edition 1769); Gr. M. van Esbroeck, Une homélie inédite ephrémienne sur le Bon Larron en grec, géorgien et arabe: AB 101 (1983) 327–362 (texts); M.R. James, Latin Infancy Gospels, Cambridge 1929, 120–126; M. Geerard, Le bon larron. Un apocryphe inédit, in FS J.H. Frede, W. Thiele, ed. R. Gryson, Vetus Latina, 24,2, Freiburg 1993, 355–363 (text Latin).
Starowieyski, M. (2014). Thief, Good. In A. Di Berardino & J. Hoover (Eds.), J. T. Papa, E. A. Koenke, & E. E. Hewett (Trans.), Encyclopedia of Ancient Christianity (Volume 3, page 781). Downers Grove, IL: IVP Academic; InterVarsity Press.
However, this History of Joseph of Arimathea, despite the name, appears to be of a later date:
"NARRATIVE BY JOSEPH OF ARIMATHEA — A non-canonical, medieval text in the Pilate Cycle that claims to report on the trial and death of Jesus in the pseudonymous, first-person voice of Joseph of Arimathea (Klauck, Apocryphal Gospels, 99). This text was never widely authoritative in the Early Church period, nor could it have been based on its late date.
The Narrative by Joseph offers details on circulating legends of Joseph of Arimathea in the medieval period, as does the earlier Acts of Pilate, in which Joseph figures as a protagonist along with Nicodemus. The narrative was likely composed in the sixth century (Burke, Secret Scriptures, 82); the earliest version of the manuscript is from the twelfth century (Allen, "Secret Disciples," 156).
In the Narrative by Joseph, Joseph is allegedly imprisoned for burying Jesus, just as he is in the Acts of Pilate, where he, along with Nicodemus, is also a key figure. In the Narrative, however, the legend reveals the imprisoned Joseph visited by Jesus and Dismas, the penitent thief, who all together journey to Galilee."
Wolcott, C. S. (2016). Narrative by Joseph of Arimathea. In J. D. Barry, D. Bomar, D. R. Brown, R. Klippenstein, D. Mangum, C. Sinclair Wolcott, . . . W. Widder (Eds.), The Lexham Bible Dictionary. Bellingham, WA: Lexham Press.
- Also, why is his feast day March 25?
- Shouldn't it be on Good Friday?
I feel like the Eastern Orthodox got this correct. I'm not sure if this is a sin or not but I agree with them over the Roman Catholics on his feast day. He died with Jesus.
It is no sin to disagree with the feast day. The Latin rite just doesn't have moveable feast days for saints and it would conflict with one of the holiest days of the Church year.