Matthew records that immediately after the death of Jesus, many Old Testament saints who had died came back to life, went into Jerusalem, and appeared to many people.
51 And behold, the curtain of the temple was torn in two, from top to bottom. And the earth shook, and the rocks were split. 52 The tombs also were opened. And many bodies of the saints who had fallen asleep were raised, 53 and coming out of the tombs after his resurrection they went into the holy city and appeared to many. Matthew 27:51-53 NASB
I think it is interesting that this occurred at the death of Jesus and not the resurrection, and I'm not sure what the reason for this could be. My main question though is what the significance was of the Old Testament saints coming back to life physically at this time. This appears to be a sort of "Zeroth" resurrection (borrowing from the Zeroth Law of Thermodynamics, that was asserted after there was already a first and second).