The body of St. Stephen, the first martyr, with those of some other saints, was discovered near Jerusalem in the year 415. Later the body of the saint was taken to Constantinople and finally to Rome, where it now lies beside that of St. Lawrence. The many miracles that occurred on the finding of St. Stephen’s body led to the institution of this festival, besides that of December 26th. A tradition states that Gamaliel, the teacher of St. Paul, had buried the body of St. Stephen at his country house; Nicodemus and Abibas, son of Gamaliel, and Gamaliel himself, were also buried there.
(Source: The New Roman Missal, Rev. F. X. Lasance)