Matt 27:50-53

This historic event is unique to Matthew's account. Graves were opened on our Thursday [Wednesday, Mark 14:12-17], and those resurrected on that morning when Christ was resurrected.
John Gill cited the prophet Isaiah 26:19, "Thy dead men shall live, together with my dead body shall they arise. Awake and sing, ye that dwell in dust: for thy dew is as the dew of herbs, and the earth shall cast out the dead."

 

Just because something is mentioned only once or only in one book or only a few times does not call into question its validity so long as the text is considered inspired, rather than an addition or modification by someone else.

 

This scripture proves that Jesus said who he was the Son Of God, gives validity to the statement... What if it wasn't there?... What if it didn't happen?... Are you sure your Salvation is intact?... Because the saints arose, gives me hope that I will too... God put his stamp on it and a ancient writer clarified the event... Salty just one more verse would have made it better... Brother Glen

Matthew 27: 54 Now when the centurion, and they that were with him, watching Jesus, saw the earthquake, and those things that were done, they feared greatly, saying, Truly this was the Son of God.

 

From Spurgeon's Matthew: the Gospel of the Kingdom (published these days by Banner of Truth).

'Christ's death was the end of Judaism: The veil of the temple was rent in twain from the top to the bottom. As if shocked at the sacrilegious murder of her Lord, the temple rent her garments, like one stricken with horror at some horrendous crime. The body of Christ being rent, the veil of the temple was torn in twain .... Now was there an entrance made into the holiest of all by the blood of Jesus; and a way of access to God was opened for every sinner who trusted in Christ's atoning sacrifice.

See what marvels accompanied and followed the death of Christ' The earth did quake, and the rocks rent; and the graves were opened. Thus did the material world pay homage to him whom man had rejected, while nature's convulsions foretold what will happen when Christ's voice once more shakes, not the earth only, but also heaven.

These first miracles wrought in connection with the death of Christ were typical of spiritual wonders that will be continued till he comes again - rocky hearts are rent, graves of sin are opened, those who have been long dead in trespasses and sins, and buried in sepulchres of lust and evil, are quickened, and come out from among the dead and go unto the holy city, the new Jerusalem.'