What does Acts 2:20 mean?
In the book of Joel, God compares a devastating infestation of locusts to the judgment He will place Judah under if they don't repent and follow Him. He warns that the "day of the LORD" will come upon them like a terrifying army that burns everything in its path (Joel 2:1–11). If they repent, God will forgive (Joel 2:12–17) and bless (Joel 2:18–27) them. Once the people have decided, God will separate them, either giving them the ability to see visions and prophesy (Joel 2:28–29) or exposing them to horrible disaster (Joel 2:30–31).Peter and other Galilean Jesus-followers have just received the Holy Spirit and the ability to speak in different languages (Acts 2:1–4). Peter equates this ability with God's promise that He will send His Holy Spirit to enable His servants to prophecy (Joel 2:28–29; Acts 2:17–18). The second part of Joel's prophecy, however, doesn't seem to apply to the early church age. In fact, very little prophecy in the Bible applies to the church age. It is often foreshortened so that a comment about the return of the Jews from Babylonian captivity is mixed with promises about the millennial kingdom. The visions, dreams, prophecies, and coming of the Spirit is happening as Peter speaks, while the blood, fire, and darkness will happen during the end times.
In Revelation 6:12, John describes how he witnessed the sixth seal of the tribulation: "the sun became black as sackcloth, the full moon became like blood." Later, during the fourth trumpet judgment, "the sun was struck, and a third of the moon, and a third of the stars, so that a third of their light might be darkened" (Revelation 8:12).
These events did not happen in the generation of the people listening to Peter. In biblical prophecy, there is a gap between the fall of Jerusalem and the temple in AD 70 (Mark 13:2) and the rapture of the church (1 Thessalonians 4:13–18). But Peter's audience understands the core truth: if they want to escape God's wrath, whether that be in life or after death, they need to repent (Acts 2:21).