In the Tridentine tradition, today is know as "The Invention of St Stephen, Protomartyr." St Stephen we know. His feast is celebrated on 26 December, when his glorious death at the hands of Jews "stiffnecked and uncircumcised in heart and ears" is commemorated. So why a second feast? Because this is the day when (as Christian tradition has it) his relics were found. (In Middle English, and even today's English, "invention" can mean "finding" or "discovery.")
Yet it is not just St Stephen's relics that were found, but (not surprisingly) those also of some Jews who were known to speak well of the Christ and Christians. Here is how the Anglican Breviary recounts the day:
It is related that, for a long time, there lay hidden, in a mean and obscure place near Jerusalem, the bodies of the following Saints : Stephen the Protomartyr ; Gamaliel, the celebrated Doctor of the Old Law, who withstood his fellow-members of the Sanhedrin when they desired to put the Apostles to death ; Nicodemus, who had defended Jesus in the Sanhedrin, and afterward came to him by night, unto whom was thereupon revealed the mystery of baptismal regeneration, and who assisted in the burial of Jesus ; and Abibon, the son of Gamaliel. Today's feast is a commemoration of the finding of the holy relicks of these men of God. ... [I]n the places where a feast is kept in honor of Saint Nicodemus, or the other aforementioned holy ones, it is observed on this day.
It appears, then, that the Western Church has not traditionally shied away from venerating these Jewish men of God who did not deny, but confessed the Christ. Let us now praise men of renown, and our fathers in their generation. (Eccl 44.1)