[2:1] On the day of Pentecost * all the believers were meeting together in one place. (Ac 2:1 NTLse)
….The purpose of prayer is not to convince God to change your circumstances but to prepare you to be involved in God’s activity.
The fervent prayer of the people at Pentecost did not induce the Holy Spirit to come upon them. Prayer brought them to a place where they were ready to participate in the mighty work God had already planned….
…The day of Pentecost arrived…Prayer had prepared the disciples for their obedient response…
…Prayer is designed to adjust you to God’s will, not to adjust God to your will. If God has not responded to what you are praying, you may need to adjust your praying to align with God’s agenda. Rather than focusing on what you would like to see happen, realize that God may be more concerned with what He wants to see happen in you.
 Once when he was eating with them, he commanded them, “Do not leave Jerusalem until the Father sends you the gift he promised, as I told you before.  John baptized with * water, but in just a few days you will be baptized with the Holy Spirit.” ( Ac 1:4-5 NLTse)
 During this time, when about 120 believers * were together in one place, Peter stood up and addressed them.  “Brothers,” he said, “the Scriptures had to be fulfilled concerning Judas, who guided those who arrested Jesus. This was predicted long ago by the Holy Spirit, speaking through King David.  Judas was one of us and shared in the ministry with us.”
 (Judas had bought a field with the money he received for his treachery. Falling headfirst there, his body split open, spilling out all his intestines.  The news of his death spread to all the people of Jerusalem, and they gave the place the Aramaic name Akeldama, which means “Field of Blood.”)
 Peter continued, “This was written in the book of Psalms, where it says, ‘Let his home become desolate, with no one living in it.’ It also says, ‘Let someone else take his position.’ *
 “So now we must choose a replacement for Judas from among the men who were with us the entire time we were traveling with the Lord Jesus–  from the time he was baptized by John until the day he was taken from us. Whoever is chosen will join us as a witness of Jesus’ resurrection.”
 So they nominated two men: Joseph called Barsabbas (also known as Justus) and Matthias.  Then they all prayed, “O Lord, you know every heart. Show us which of these men you have chosen  as an apostle to replace Judas in this ministry, for he has deserted us and gone where he belongs.”  Then they cast lots, and Matthias was selected to become an apostle with the other eleven. ( Ac 1:15-26 NLTse)