Was Joel's prophecy of the Holy Spirit being poured out fulfilled at Pentecost?
Yes. Peter, quoting Joel word for word, declared at Pentecost, "this is that which was spoken of Joel the prophet." The prophecy that God would pour out His Spirit upon all flesh had happened on that day; these things had come to pass.
Les explains that Peter in Acts chapter 2 is in the same mode of language Jesus used, quoting the Old Testament prophet Joel. Peter responds to the accusation that the Galileans were drunk by saying it was only nine o'clock in the morning, and then he declares, "this is that which was spoken of Joel the prophet." He quotes Joel word for word about the last days, God pouring out His Spirit upon all flesh, sons and daughters prophesying, young men seeing visions, old men dreaming dreams, and servants and handmaidens receiving the Spirit. Les notes that Joel wrote prophecy of a future day, then asks whether this had happened on the day of Pentecost, answering, "Sure. These things have happened." Thus the context affirms the fulfillment at Pentecost.