According to Acts, Jesus’ resurrection inaugurated his messianic reign, and his giving of the Holy Spirit inaugurated his rule through the Christian church (Acts 1:8; 2:1-43). The events at Pentecost (Acts 2) were understood by Peter as a fulfillment of Joel’s prophecy concerning the last days where God’s Spirit would ascend upon all of God’s people (Joel 2:28 cf. Acts 2:15-17). G.K. Beale writes,
The reason why the coming of the Spirit is perceived in such a highlighted eschatological manner is that one of its purposes was to demonstrate the exalted, heavenly messianic kingship of Jesus, as a result of the resurrection from the dead. This was natural because the spirit was linked with the future hope of resurrection life in the OT and in Judaism.
Moreover, Acts portrays a future expectation and fulfillment of the “latter days.” Acts 1:6-7 depicts Jesus’ disciples asking “Lord, are you at this time going to restore the kingdom to Israel?” (v. 6), and Jesus responded, “It is not for you to know the times or dates the Father has set by his own authority” (v.7). Acts 3:21 refers to Jesus’ ascension and return by stating, “Heaven must receive him [Jesus] until the time comes for God to restore everything, as he promised long ago through his holy prophets.” Lastly, the Apostle Paul in Acts 17:30-31 and 26:6-7 refers to a future day of judgement at the end of history and to “a hope of the promise of a final resurrection for the nation of Israel.”
 G.K. Beale, A New Testament Biblical Theology: The Unfolding of the Old Testament in the New (Grand Rapids, MI: Baker Academic, 2011), 136-137.  Beale, A New Testament, 137.  Beale, A New Testament, 138.