Matthew 27:53

And came out of the graves after his resurrection - The narrative of Matthew does not determine whether they came to life before Jesus rose, and remained in the tombs, or came to life after he died. The latter is probably the correct opinion. There is nothing said of the reason why they were raised. It is not improbable to suppose that it was, amid the other wonders attending the death of Jesus, to convince the Jews that he was the Messiah. Perhaps some who had been his open friends were raised up now as an attestation that he in whom they had believed was the Christ. What became of them after they had entered into the city whether they again died or ascended to heaven, is not revealed, and conjecture is vain.

The holy city - Jerusalem, called holy because the temple was there, because it was devoted to God, and because it was the place of religious solemnities.