Acts 2:17-18 And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall visions, And your old men shall dream dreams: and on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy.
On the Day of Pentecost, after Jesus’ Resurrection and Ascension, Jesus poured out the Holy Spirit on the 120 disciples gathered in Jerusalem. Peter proclaims that this event fulfills the prophecy of Joel. The promise of a poured out Spirit of God includes us as well, for he says ”all flesh.” Since Jesus finished His Salvation Work and sat at the Father’s right hand, He did, and He continues to, pour out the Spirit. He died for all; now He sends the Holy Spirit upon all.
To make the point that the Holy Spirit is available for all kinds of people, he includes male and female, old and young, rich and poor, slave and free as examples that no human person is excluded from this promise. And this will happen ”in the last days,” which includes all the time of history from the Ascension of Christ to the Second Coming.
The What is the promised indwelling of the Spirit; the Who is all people; the When is all the days in which we are living since then; the Where is everywhere to the ends of the earth; the Why is Jesus’ finished work: the How will manifest in various ways with Salvation as the result. The promise is for you and your children.