Just as in God's genius Easter completes the meaning of Passover, so does the Coming of the Holy Spirit complete Pentecost's Giving of the Law.
The Jewish Pentecost, 50 days after Passover, marks the gift of the law, which made the people of God a moral and ethical people, mirroring their Creator, separated from the pagans around them. The Holy Spirit was and is, in turn, our ultimate guide, companion, and revealer of the Godhead.
We can truly know God only through the Holy Spirit. Our senses alone are inadequate in revealing God.
The Holy Spirit is God's seal on the Christian -- the indwelling of the Spirit marks the true guarantee of our salvation.
If we look to Easter as the happy ending of our faith, we miss essential aspects of what it truly means to be a Christian. Jesus' death and resurrection, YES!, but followed by his ascension, his return to his heavenly glory, and then his sending the gift of the Holy Spirit, our great Comforter, and completion of the Triune God -- are all essential aspects of our theology.