When did Jesus send the Holy Spirit?

I've always assumed that the Holy Spirit did not make his presence known amongst men until Pentecost. Famously, in Acts 2, the Spirit descends upon the first believers, gives them power of speech, and adds tremendously to their power. Jesus clearly foretells the coming of the Holy Spirit in Acts 1 - just before he ascends into heaven - saying: >But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.” After he said this, he was taken up before their very eyes, and a cloud hid him from their sight. That Jesus *must* go away before the Holy Spirit comes is attested to in John 16:7 >But very truly I tell you, it is for your good that I am going away. Unless I go away, the Advocate will not come to you; but if I go, I will send him to you. All of this fit my narrative neatly, thinking, okay - Jesus takes center stage until He returns to the Father, at which point the Holy Spirit comes onto the scene with tongues of fire and such. Sure, he may have "descended on Jesus like a dove" at Jesus' baptism, but he was pretty much quiet in the affairs of men. And indeed, as an eternal pre-existent being much like Jesus, I assume he was around, just not so much involved in the day to day affairs of men. But then, I listened closely to the lectionary this week. I heard something in John 20:22 that I was not expecting. After Jesus appeared to his disciples, after the resurrection, he said: >And with that he breathed on them and said, “Receive the Holy Spirit." This seemingly contrasts with a non-involved Holy Spirit. I thought that Pentecost was the "firstfruits of the spirit," the first time the Holy Spirit deigned to dwell amongst men. Am I off my rocker, or am I missing something? In others - was - a. Pentecost supposed to be the first appearance of the Holy Spirit, or - b. am I just making up the idea that Pentecost was supposed to be the *first* outpouring of the Spirit? If (a), how is John 20:22 reconciled? If (b) how could the Holy Spirit come in light of John 16:7? In other words, when did Jesus send the Spirit, and how does it fit with the external witness?