Joel 2:28, 29 says, "It shall come to pass afterward, that I will pour out my Spirit upon all flesh." Actually, this passage pertains to both the Jews and the United States--it has what is called a dual application prophecy. The reason I know this is because Peter applies this to the Day of Pentecost in Acts 2:16-18.
He specifically quotes Joel, and says that Pentecost is a direct fulfillment. However, he does not say that it is the only fulfillment. We know this because God is going to pour His Spirit on His people just before the "dreadful day" of the Lord (in Malachi 4), which is of course the judgment day. People will be filled with the Spirit and have the same kind of power as Elijah.
This is what the Bible called the former rain. And there will be a latter rain to follow. Farmers planted their seeds, and the former rain germinated the seed, and it would grow through the season. Just before the harvest, there is a latter rain that "plumps" the crop before harvest, so to speak. And in the same way in Acts, the former rain fell from heaven.
Just before Jesus comes, He will again drench His people with the Holy Spirit, that they might preach the way the apostles did. There'll be power and multitudes converted prior to the harvest when Jesus comes. And Revelation pictures Jesus coming to harvest.