Pentecost was and is a Jewish holiday that occurs fifty days after Passover (hence the name “pente” which means fifty). This is the day the Apostles are indwelled by the Holy Spirit. The period of spiritual preparation after the dramatic events of the crucifixion and resurrection has ended and the time of evangelization has begun. It is considered the birthday of the Church. One of the reasons that this days is likely chosen by God is because it brings the pilgrims back to Jerusalem. Thus, there will be great crowds again and many people who were in the city and witnessed the crucifixion will have returned.
Notice, it is Peter who gives the first sermon. It is Peter who announces the good news to the world. The Apostles are given the gift of tongues so that when they speak everyone who hears can understand. This is a clear sign that the gospel will be preached to the whole world, Jews and Gentiles alike.
Today we read the first half of Peter’s message. There are some interesting things in what he said. Peter preaches the God promised:
And I will show wonders in the heavens above
and signs on the earth below,
blood, and fire, and vapor of smoke;
the sun shall be turned to darkness
and the moon to blood,
before the day of the Lord comes, the great and magnificent day.
These things actually happened on the day of the crucifixion. On the day of the crucifixion the gospels record that the sky darkened. On great video that you can watch online called, The Star of Bethlehem, traces the origin of the star of Jesus’ birth. It uses astronomy software to make a strong case as to what the star was. It also uses this software to look at the sky on the day of the crucifixion. It shows a total eclipse of the sun on Good Friday and the rising of blood red moon that evening. The gospels also record an earthquake that was strong enough to crack the floor of the temple.
Thus Peter’s sermon is not just a vague generalization about some signs that are to come. What he is actually saying to the Jews assembled in Jerusalem for the feast is, ‘You know it happened 50 days ago when you were last here and Jesus of Nazareth was crucified. Now you know who He is.”