J.C. Fenton (Westminster Pelican Commentaries: Saint Matthew, Philadelphia: Westminster Press, 1963, page 444) comments:

Matthew has used Mark 15:38 for 27:51a, and Mark 15:39 for 27:54; so far as we know, he had no written source for vv. 51b-53.

W.F. Albright and C.S. Mann (The Anchor Bible: Matthew, New York: Doubleday, 1971, pages 351-352) write:

It is certainly no service to scholarship to find in these verses an imaginative piece of fiction on the part of the evangelist, or simply an attempt to garnish the account of the passion with improbable details. Enough has surely become known in the course of the past fifty years about the language and forms of apocalyptic to evaluate this material seriously. W.G. Essame (Matthew xxvii 51-54 and John v 25-29," ET 76 [1964-65], 103) is certainly correct in seeing here a dramatization of a saying preserved in the Johannine tradition. It is worthy noting that Essame is following the lead of a much underrated NT scholar, W.K. Lowther-Clarke, who saw the verses as a triumphant assertion in OT language that the resurrection of Jesus was a divine act. The whole complex of these verses is reminiscent of the triumph of the saints described in Dan vii 18, 21, 22, 25, and 27...

51. the curtain of the Most Holy Place. Matthew and Mark report this detail. In the absence of any firm dating for the crucifixion from external sources, coupled with a lack of historical evidence for an earthquake coincidental to the crucifixion, it is not possible to say whether this detail was intended to be read as history, or whether by this means the evangelists are further pursuing symbolism. Eph ii 14 is the Pauline assertion that in the community of the New Covenant Israel now embraces both Jew and Gentile, while Heb ix 8 ff. expresses the way "opened by the sacrifice of the cross." The position of the saying in Matthew certainly seems to indicate a symbolic meaning. Josephus (Jewish War VI. 299) has an account of an earthquake before the fall of Jerusalem, while a letter of Jerome (120.8) recalls that the now lost Gospel according to the Hebrews speaks of a cleavage in the masonry of the temple porch, which might have left the Most Holy Place open to view. The Talmud (TB, Yoma 39b) has an interesting story concerned with Rabbi Yhanan be Zakkai, which reports that the doors of the temple opened of their own accord forty years (sic) before the fall of Jerusalem, so portending the end of the temple.

Willoughby C. Allen (International Critical Commentary: St. Matthew, Edinburgh: T&C Clark, c. 1910, pages 296-297) also provides considerable documentation of surrounding events.

51. And behold the veil of the Temple was rent from the top to the bottom into two.] Mk. has: " And the veil of the Temple was rent into two from the top to the bottom." ...Jos. Wars, vi. 290, records, amongst other portents that preceded the fall of Jerusalem, the following: "At that feast which we call Pentecost... the priests felt a quaking, and heard a great noise; and after that heard a sound as of a great multitude, saying, ' Let us depart hence.' " In B. Joma 39b it is said that, amongst other signs which happened forty years before the destruction of the Temple, "the doors of the Temple opened of themselves until Rabbi Jochanan ben Zaccai rebuked them, saying, '0 Temple, Temple! Why troublest thou thyself? I know that thy end is near.' " Zahn may be right in suggesting that all these accounts are reminiscences of an event that happened at the porch of the Temple at the period of the crucifixion. A cleavage in the masonry of the porch, which rent the outer veil and left the Holy Place open to view, would account for the language of the Gospels, of Josephus, and of the Talmud.

Mt. here adds:

52, 53. And the earth was shaken, and the rocks were rent; and the iambs were opened, and many bodies of the saints that had fallen asleep arose, and came out of the tombs after His resurrection; and entered into the Holy City, and were made manifest to many.] The passage probably comes from Mt's cycle of Palestinian traditions. The cause of the rending of the veil was an earthquake, which also exposed the bodies of the dead by laying bare their graves. These dead saints, whose rest was so rudely shattered, appeared to many in the city. Mt. adds this account to the Marcan record, but interpolates a clause which is inconsistent with the obvious meaning of the tradition. If Christ was the first-fruits of them that slept, how could His resurrection have been preceded by that of these saints ? Under the influence of some such idea the editor adds the caution, "after His resurrection." Or had his authority, "After their resurrection"; and did he by mistake or purposely alter "their" to "His"?

Floyd V. Filson (Black's New Testament Commentaries: The Gospel According to St. Matthew, London: Adam & Charles Black, 1971, page 297) notes,

This puzzling story may originally have been a figurative teaching, but 'Matthew' takes it as a real event.

Eduard Schweizer (translated by David E. Green, Jesus, Atlanta: John Knox Press, 1971, pages 53-54) writes:

Matthew 27.51-53 gives a remarkable account concerning the dead who arose from their graves immediately after Jesus' death and wandered about Jerusalem; this story must go back to a community or faction that looked upon the death and resurrection of Jesus as the beginning of the eschaton and obviously expected that the general resurrection was about to take place. (Matthew himself, of course, no longer interpreted the story in this way, seeing in it only a sign of the power of Jesus' crucifixion, which overcomes death.) It is salutary to suffer the shock of these strange ideas. Perceiving the "strangeness" of accustomed ideas is usually the beginning of real attention to them. Three points illustrate the salutary effect of this perspective, which seems to us so strange and alien in its conceptions:

(a) The community learned from the experiences of Easter that salvation rests completely in God's actions. Their faith had failed com- pletely. Their attitude toward what had happened had not prevailed. Their accomplishments in the field of religion or ethics were negative. But God had acted. It was a strange and mysterious act) which they could share only as witnesses. Their contribution had been nil. What took place came to them as a pure gift. God's battle cry) his archangel) and his trumpet blast underline the point so that it cannot be missed; the crucial event, the "end" that is the beginning of God's kingdom, depends absolutely upon God.

(b) The events of Easter comprehend a world. What took place then could not be calculated within the sphere of individual experience; it could only be described in terms of divine creation. What was manifested here was nothing less than the abrogation of the entire world of death) and God was restored to his proper place. This, too, is underlined by the expectations of the first community. Jesus descent from heaven, the ascent of the faithful into the air) above all the idea of a new earth under a new heaven) which originates in the Old Testament (2 Peter 3.13)—these stress the universality and concreteness of Easter, show it as an act of God transcending all that takes place merely within the human heart.

(c) From the very beginning Easter was understood as an open door to the future. Certainly none of the disciples saw in it a historical event completed in the past, able at best to exercise its influence indirectly. Many of them brought their families to Jerusalem; this shows how strongly they felt Easter to be, as it were) a continuing event. Once again eschatological expectations emphasize the pregnancy of what God did at Easter for the future. Presumably the first disciples hardly distinguished Easter from the Parousia. What took place in Jerusalem and Galilee at Easter was the beginning of God's mighty eschatological acts that destroy the old world. For them, Easter continued on, as it were, into the consummation of the kingdom of God) in which Jesus' disciples will live "with Christ" forever.

To think of Christianity as timeless is to deprive it of hope. In contrast to such a view, the first community speaks with a refreshing and perhaps terrifying simplicity of God's mighty act that comprehends a world and culminates in the consummation of God's kingdom. Here the important thing is not the conversion of individuals, powerful as that can be; what matters here is a transformed world, in which God is Lord not only of individual hearts, but of all power, of all life, of all history, so that nothing can oppose him, however much all his creatures stand apart from him and are not simply absorbed by him.

The Apostle Paul's words to the Christians in Thessalonica provide a fitting conclusion to the commentaries provided above. It should be noted that Paul wrote these words twenty or thirty years before the Gospel According to Matthew was written. In fact, some scholars have argued that the Gospel According to Matthew is partially a response to Paul's writings, to which those scholars believe the writer of Matthew had access. 1 Thessalonian 4:13-18 (NRSV) reads:

We do not want you to be uninformed, brothers and sisters, about those who have died, so that you may not grieve as others do who have no hope. For since we believe that Jesus died and rose again, even so, through Jesus, God will bring with him those who have died. For this we declare to you by the word of the Lord, that we who are alive, who are left until the coming of the Lord, will by no means precede those who have died. For the Lord himself, with a cry of command, with the archangel’s call and with the sound of God’s trumpet, will descend from heaven, and the dead in Christ will rise first. Then we who are alive, who are left, will be caught up in the clouds together with them to meet the Lord in the air; and so we will be with the Lord forever. Therefore encourage one another with these words.