Well, Matthew 27:52-53 states that during the earthquake at Jesus' death, "the tombs opened and many bodies of holy people (polla sómata tón hagión) who had died [lit. "sleeping (in death") were raised (égerthésan), and these, after his resurrection, came out of the tombs, entered the Holy City and appeared (enephanisthésan) to many people." The verb egeiró "rise up" can have mundane senses (cf. Mark 2:9, 4:27, John 11:29, Acts 9:8, 12:7, Revelation 11:1), but when it is used to refer to the dead rising, it always refers to resurrection....within Matthew and outside of the gospel (cf. Matthew 10:8, 11:5, 16:21, 17:9, 23, 26:32, 27:63-64, 28:6-7, Mark 12:26, 14:28, 16:6, Luke 7:14, 22, 9:22, 20:37, 24:6, 34, John 2:19-20, 22, 5:21, 12:9, 17, 21:14, Acts 3:15, 4:10, 5:30, 10:40, 13:30, 37, 26:8, Romans 4:25, 6:9, 7:4, 8:11, 34, 10:9, 1 Corinthians 15:4, 12-17, 20, 32, 35, 42-44, 52, 2 Corinthians 1:9, 4:14, 5:15, Galatians 1:1, Colossians 2:12, 2 Timothy 2:8, Hebrews 11:19, 1 Peter 1:21, etc.), and similarly the verb emphanizó "appear, manifest," which occurs in Matthew 27:53 concerning the dead "appearing" in Jerusalem, is used in Acts 10:40 to refer to Jesus post-resurrection appearances, and the related forms phainó (cf. Mark 16:9, Luke 24:11) and phaneroó (cf. Mark 16:12, 14, John 21:1, 14) are regularly used to refer to Jesus' post-resurrection appearances. Taken together, the language is strongly suggestive of resurrection.
The idea that Jesus' resurrection was accompanied with a resurrection of others released from Hades (i.e. the communal resurrection), or was preceded by a preaching to the faithful dead in the underworld, appears in Ephesians 4:8-9, 1 Peter 3:18-19, Odes of Solomon 17:9-16, 22:1-10, 42:3-20, Justin Martyr, Dialogue 72.4, Irenaeus, Adversus Haereses 4.27.1-2, 4.33.12, 5.31.1, Sibylline Oracles 8.310-317, Epistula Apostolorum 51, and probably the Gospel of Peter 10:39-42.
However the statement in Matthew (which doesn't refer to any ascension of these resurrected ones) directly contradicts Paul's doctrine on the resurrection. 1 Corinthians 15:20-23 states: "But Christ has in fact been raised from the dead, the first-fruits of all who have fallen asleep. Death came through one man and in the same way the resurrection of the dead has come through one man. Just as all men die in Adam, so all men will be brought to life in Christ; but all of them in their proper order: Christ as the first-fruits and then, after the coming of Christ, those who belong to him." Matthew 27:51-52, by stating that "the bodies of many holy men rose from the dead" at the very moment Jesus died suggests that the resurrection of the righteous happened before Jesus was resurrected. Here the "proper order" is reversed. Paul believes that "all who have fallen asleep", "all [who] die in Adam" (whom he equates with those "dead in Christ," since they have been redeemed by Him) would rise at Christ's return. This is the same view expressed in 1 Thessalonians 4:16, where those "dead in Christ" (presumably all who have been redeemed by Christ) "will be the first to rise" at Christ's second coming. 2 Timothy 2:18 (of the Paulinist school) specifically refutes the notion that "the resurrection has already taken place."
So what's the deal with Matthew's narrative? It is important to note that this story about the holy ones rising from the dead was a Matthean addition to the original Markan account. This story is absent in Mark 15:38, and it doesn't appear in the Lukan version of the account (Luke 23:45). In fact, other early gospels based on the same crucifixion narrative also omit any mention of this resurrection but mention the splitting of the temple veil (cf. Gospel of Peter 5:15-20; Gospel of the Nazoreans, Fr. 21, 36; Acts of Pilate 11:1-3). Now this in itself is quite remarkable since the resurrection of the holy ones would have been an awesome event, even more so than the tear in the temple veil, yet it is nowhere mentioned by Mark, Luke, John, or any other gospel except of Matthew and later versions of Matthew. Furthermore, it is fairly certain that the account in Matthew has been altered in its wording. The biggest problem are the words "after his resurrection", which -- most bizarrely -- delays the appearance of those resurrected for three days (Matthew 27:53). This totally defeats the whole purpose of having them raised when Jesus dies on the cross as something that led the centurion to confess Jesus as the Son of God (Matthew 27:54). The centurion certainly could not have been awed by their resurrection if the resurrected dead did not leave their tombs! The delay however does bring the canonical account into line (partially) with Pauline theology, which proclaims Jesus as the "first fruits" of the resurrection. For Paul, the "saints" could not arise before Jesus himself has risen. Yet the gloss fails to bring Matthew fully into line with Pauline theology since the bodily resurrection of the righteous dead still takes place before Jesus.
There is actual textual evidence that the text in Matthew has been altered. One of its earliest witnesses was the Diatesseron, a gospel harmony produced by Tatian in the late second century. This harmony was in turn based on the one produced by Justin Martyr several decades earlier. The Pepysian Harmony and the Ephrem Commentary both attest the Diatessaron reading as follows:
And with that, the veil that hung in the temple before the high altar burst in two pieces, the earth quaked, and the stones burst, and the dead men arose out of their graves. And entering the holy city, they appeared to many. And the centurion and those with him, who stood facing Jesus, saw the earthquake and what took place, and said with awe, "Truly this was the Son of God!"
Here the interpolated gloss does not appear and the appearance of the risen dead in Jerusalem occurs at the same time as Jesus' death and was witnessed by the centurion. This reading makes better sense with the context. It also lacks the greater detail of the canonical account in this passage -- all of which is theologically loaded: "bodies" (more specific and agreeing with Paul in 1 Corinthians 15:35-44), "of saints" (certainly superior to the mere "dead" of the Diatessaron, and therefore more developed, and also Pauline), "who had fallen asleep" (again a more elegant description, and again used by Paul in 1 Corinthians 15:20 and 1 Thessalonians 4:14). This suggests that the Diatessaron's reading is earlier and preserves a more primitive version of the text than does the canonical text, which has been revised to bring it into conformity with Pauline theology.