In our Western culture we often think of things in a linear fashion. When we read a book or watch a movie, the events normally happen in order, three follows two follows one, etc. But there are movies and books that jump around in time. One event happens and then a flash-back (or flash-forward) jumps us to a different part of the story. What makes prophecy difficult to understand is that it does exactly that. I think we see a good example of this in Joel chapter two. Joel 2:1-2 talks about The Day of the Lord and thick darkness. The Day of the Lord is a period of time in the future where God defends Israel from the nations that have gathered against her. We know from Jesus conversation in Acts 1:6-7 that the restoration of Israel happens at some point in the future. And we know from history that Israel has not been fully restored yet, although I believe that time is approaching. We also know that Joel 2:28 was fulfilled in the past. This happened on the Day of Pentecost in Acts 2:1-17.
This entire chapter is talking about the end times (which include the first and second coming of Jesus). In general it speaks of the Day of the Lord, but then it jumps back to Acts chapter two and mentions that this entire period would be a time when God's Spirit is poured out. Peter claimed this began in his time. Pentecost is still here...God's Spirit still guides us.