(Act 2:1 ESV) When the day of Pentecost arrived, they were all together in one place.
Why Pentecost? God chose the time, and so it can’t be a coincidence. What’s special about Pentecost? Well, two things.
First, Pentecost is a celebration of first fruits. The Wikipedia describes the ceremony —
The Bikkurim [first fruits] were brought from the Seven Species for which the Land of Israel is praised: wheat, barley, grapes, figs, pomegranates, olives, and dates (Deut. 8:8). In the largely agrarian society of ancient Israel, Jewish farmers would tie a reed around the first ripening fruits from each of these species in their fields. At the time of harvest, the fruits identified by the reed would be cut and placed in baskets woven of gold and silver. The baskets would then be loaded on oxen whose horns were gilded and laced with garlands of flowers, and who were led in a grand procession to Jerusalem. As the farmer and his entourage passed through cities and towns, they would be accompanied by music and parades.
The celebrants would have been there since at least the week before Passover (the lamb to be sacrificed was selected a week before Passover). Jews would have been there from across the Roman Empire. Travel was expensive and dangerous, and so for many, this was a once-in-a-lifetime pilgrimage — their one Passover in Jerusalem and their one Pentecost. And so Pentecost itself was probably the conclusion of their trip — and a party!
There would have celebrations, parades, and feasts. Farmers from the surrounding Judean countryside would have brought in their sacrifices, filling the city with fruit and grain.
Like most ancient cities, Jerusalem had narrow, winding streets. And it was built on top of two mountains. People would have been packed in the city shoulder to shoulder, pushing through the crowds to reach the Temple or just to find a good view of the happenings.
The Jews also believed that God gave the Torah to Moses on Mt. Sinai on Pentecost (and it’s a reasonable but not certain reading of Exodus). Therefore, Pentecost was an especially holy day to the First Century Jews. Pentecost celebrates the day they entered into a new covenant relationship with God (Exo. 24).
God chose Pentecost, therefore, because —
* There were Jews present from around the Empire, Judea, and Galilee, assuring an accelerated spread of the gospel.
* The first converts would be the first fruits of the new covenant.
* God would have a new covenant with his people.
* Pentecost was celebrated in Jerusalem, which is where the prophets promised the Exile would end.
* Pentecost is a time of celebration of God’s good works, his blessings on his people.
Luke reports later in the chapter that 3,000 were converted that day. It was 3,000 men who died because they worshiped the golden calf (Exo 32:28). The number meant something to Luke’s readers. It symbolizes God’s restoring things to their right order, undoing the great idolatry of the Jews.
It goes deeper. The Exile was caused by idolatry. The Exile was ended by the coming of Jesus and the outpouring of the Spirit, which brought in the 3,000. To a Jew, the number would have been a profound message condemning Judaism as practiced by the First Century Jerusalem Jews, implying that the gospel preached by Peter would bring the promised end of the Exile and the beginning of the Kingdom.