The deacon Stephen, stoned in Jerusalem two years after the death of Christ, has always been the object of very special veneration by the faithful. He is the first martyr. The account in the Acts of the Apostles relating his arrest and the accusations brought against him emphasize the parallel with our Saviour's trial; he was stoned outside the city wall and died, like his Master, praying for his executioners.

Stephen belongs to the group of seven deacons whom the Apostles associated with their work in order to lighten their load. He was "filled with faith and with the Holy Spirit," "full of grace and strength" he showed himself as a man of God, radiating divine grace and apostolic zeal. As the first witness to Christ he confronted his opponents with quiet courage and the promise made by Jesus (Mark 13.11) was fulfilled: ". . .Disputing with Stephen they were not able to resist the wisdom and the spirit that spoke."

In St. Stephen, the first martyr, the liturgy emphasizes the imitator of Christ even to the extent of the complete gift of self, to the extent of that great charity which made him pray in his suffering for his executioners. By establishing the feast on the day after Christmas the Church draws an even closer comparison between the disciple and the Master and thus extends his witness to the whole mission of the redeeming Messiah.

http://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2014-12-26