Matthew is the only Gospel writer who records the incident of the raising of "holy people" (Matthew 27:50-53) at the time of Jesus' death. The identity of those who were raised is unknown. They obviously were men and women who had followed God closely and who were raised to bear witness to the effectiveness of Jesus' sacrifice for our sins on the cross and the power of his subsequent resurrection.
It appears that they were raised at the time that Jesus died, but did not appear to people until after His resurrection. Also, since Christ is "the firstborn from among the dead" in the sense of being the first to be raised with a transformed resurrection body (Colossians 1:18, 1 Corinthians 15:20), it would seem that these others were like Lazarus (John 11), raised to an ordinary human life subject to dying again.