The passage refers to three different time periods: Verse 17 – “the last days”; Verse 18 – “in those days”; Verse 20 – “before that great and notable day of the Lord come”. Peter is quoting from Joel 2:28-32 as a prophesy partially fulfilled on the day of Pentecost.

The part fulfilled was that the spirit would be upon the “servants and handmaidens”, and “they would prophesy.” They miraculously spoke in other known languages to spread God’s word. Note verse 6, “every man heard them speak in his own language,” verse 8 “everyman in our own tongue,” and verse 11 “do hear them speak in our tongues.” Verses 9-11 list the different languages they spoke.

God does everything for a purpose. At that time, Jews from around the Mediterranean region were coming to theTemple for yearly worship. What did they hear? Verse 11 “the wonderful works of God” in their own language to give them understanding. That very day the Christian Church was born. God’s timing created the opportunity for the Gospel to be spread to a very broad audience

Jesus said the spirit would “guide you into all truth” (John 16:13), and “He shall give you another Comforter that he may abide with you forever even the Spirit of truth.” (John 14:16-17) Thus, the holy Spirit guided these new Christians to apprehend the critical doctrine that Jesus was the Messiah. Understanding that salvation was through Him alone was a work of the Spirit.

A different work of the spirit, Acts 2:17, will be in the last days and “upon all flesh.” Obviously, this has not yet happened. If God’s holy Spirit had been poured out upon the world, righteousness, love, and justice would flourish. Sadly, greed, injustice, and deceit are still the controlling forces in the world. 

Jesus is in the process of tearing down and eradicating this present, evil order. Daniel 2:44 (ESV), “And in the days of those kings, the God of heaven will set up a kingdom that shall never be destroyed,…It shall break in pieces all these kingdoms and bring them to an end, and it shall stand forever.” This is in preparation for the spirit to be poured out on all.

Verse 20 pictures the holy Spirit working upon events in nature. These have both a literal and symbolic application. “The powers of heaven” refer to spiritual ruling powers, both to spirit beings and religious institutions. Recall that in Ephesians 2:2, Satan is called the prince of the air. “Signs in the earth” include the destruction of earthly governments and natural disasters like earthquakes. (See Zechariah 14:3-4 which primarily is symbolic, but also may have a natural fulfilment.)

Following the context of Acts 2 verse 21, the purpose of the previous events is to bring mankind to “call on the name of the Lord.” This will certainly happen during the Lord’s Second Advent.