March 25 is the traditional date of Jesus' crucifixion, and so would have been the day when the "good thief" died.
While March 25 is celebrated in the Roman Rite as the day of the Annunciation to the Blessed Virgin Mary, the Roman liturgy also has a tradition of listing a variety of "extra" saints on each day of the year.
The Roman Martyrology lists every saint that the Roman Church recognizes, even if they are not given precedence on a particular day.
The 2004 edition of the Roman Martyrology gives the following listing on March 25:
Commemoration of the Good Thief, who, having professed faith in Christ on the cross, deserved to hear from him: “Today you will be with me in paradise.”
St. Dismas
Probably the best known criminal-turned-saint in the Catholic Church is St. Dismas. He was the “good” or “penitent” thief crucified alongside Jesus, and was promised Heaven by Jesus Christ himself:
Now one of the criminals hanging there reviled Jesus, saying, “Are you not the Messiah? Save yourself and us.” The other, however, rebuking him, said in reply, “Have you no fear of God, for you are subject to the same condemnation? And indeed, we have been condemned justly, for the sentence we received corresponds to our crimes, but this man has done nothing criminal.” Then he said, “Jesus, remember me when you come into your kingdom.” He replied to him, “Amen, I say to you, today you will be with me in Paradise.” (Luke 23:39-43)
These words of Jesus Christ led the early Christian community to believe the “good thief” repented of his crimes and entered Heaven later that day. This automatically made him a saint, as a saint is more loosely defined as anyone enjoying the beatific vision in Heaven.
Very little is known about this man, though tradition often applies the name “Dismas” to him, which means either “sunset” or “death.” What can be ascertained is that he was a criminal in the eyes of Roman law.
His feast day was set to March 25 because of an earlier tradition that Jesus died on that day.
He remains one of the only individuals in the entire Bible who was promised to enter into Heaven by God himself.
St. Dismas is the patron saint of prisoners, as well as ex-convicts, showing them a path to true repentance.
