... continued from Thursday.
Peter's sermon thus serves as a kind of encapsulation of the gospel. It gives us a snapshot of the message of Acts. It gives us an overview of what the earliest Christians preached.
The first part of each sermon often reached back into the Old Testament Scriptures and showed that Christ's death was all part of God's plan. In the Acts 2 sermon, Peter even uses Scripture to show that the Day of Pentecost was part of God's long term plan. Acts opens up Joel 2:28-32 through the eyes of the Spirit and hears an overtone of what was taking place that day.
Joel 2 was originally about a locust disaster that had ravished Israel (Joel 2:25). "Armies" of locusts had destroyed the land, bringing a time of gloom and darkness (2:2). The words that Peter quotes originally had to do with God's promise to restore the land of Israel after that horrible plague.
But more than one New Testament author also heard in these words a promise of restoration that was taking place in their day as well with Jesus and the Spirit.  If Israel would repent of its sins, God would refresh the land with his Spirit. Everyone who called on the name of the Lord would be saved (Joel 2:32).
While Joel 2 originally referred to God the Father (Yahweh) as the Lord, the early Christians called Jesus "Lord" and applied this verse to calling on Jesus for salvation (e.g., Rom. 10:13). In Romans, salvation is escape from the wrathful judgment of God that was soon coming on the earth (Rom 1:18). We can see how it worked at least in Paul's mind. 
God's judgment was coming on the earth. Through his death and resurrection, God had used Jesus to make it possible to escape God's wrath--to be "saved" and experience "salvation." Those who called on Jesus the Lord could be saved and would receive the Holy Spirit as a sign of these earth-shattering events.
A couple other aspects of Joel 2 are interesting. We see one of them in an expression I just used, "earth-shattering."  No native English speaker will think that I am literally suggesting that the earth will break apart upon hearing of "earth-shattering" news. It is a metaphor, "apocalyptic" language that indicates something incredibly significant is happening in history.
In the same way, the sun darkening and the moon turning to blood probably did not refer to any literal damage done to these celestial bodies. Peter is saying, in effect, that the Day of Pentecost is a "sun-darkening" and "moon-bloodying" event. It is an event that will alter the course of history.
One element of that history altering event is that the "daughters" of Israel will prophesy as a result of the Spirit, not only the "sons" (Acts 2:17-18). Luke seemed especially interested to show the role that women played in the story of the church, including their role as prophets (cf. Acts 21:9). The Spirit is the great equalizer of the genders, a truly remarkable idea in Luke's own day.
And why wouldn't the Spirit have this effect. The differences between men and women are a matter of their bodies. If women have the same access to the Spirit that men do, then we would expect women to have just as much spiritual wisdom to share as any man. Prophetic speech, preaching the good news--these are spiritual activities, and so we should not be surprised to find that Paul simply assumes women will prophesy and pray in public worship (1 Cor. 11:5)...
 Like so many Christian readers today, they may or may not have paid much attention to the verses about the locusts that came right before the words that especially jumped out at them. Nevertheless, Israel was under foreign rule at the time, and it would have been easy at times to compare the armies of the Romans with the armies of locusts in Joel 2.
 Luke-Acts does not have quite as strong a sense of God's coming wrath as Paul does, for example. Salvation in Luke has more to do with healing and restoration than with escaping God's wrath (e.g., Luke 18:42; Acts 4:9).
 I owe this insight to N. T. Wright, I think in his summary of the third quest for the historical Jesus in the chapter he added to Stephen's Neil's The Interpretation of the New Testament: 1861-1986. *** Wright at this point was significantly drawing on his Doctor Father, G. B. Caird, whose massively insightful The Language and Imagery of the Bible argued that we take a lot of apocalyptic language literally that was not really meant to be taken in this way.