When the Day of Pentecost had fully come, they were all with one accord in one place. --Acts 2:1, NKJV
Here they were, right where Christ commanded them to be. Waiting on the outpouring of the Holy Spirit. The secret of the coming of the Holy Spirit on the church is easy to understand when we realize that the Apostles completely obeyed Christ's comand.
The need of the day is the Spirit's fullness if we are to be effective Christians. How shall we receive it? Utter obedience.
"...And we are His witnesses to these things, and so also is the Holy Spirit whom God has given to those who obey Him." --Acts 5:32, NKJV