We should remind ourselves immediately that the Feast of Pentecost was one of the three great Jewish feasts, along with Passover and Tabernacles. The word 'Pentecost' means 'Fiftieth' and it was 50 days after Passover. At least as many Jews headed for Jerusalem to celebrate Pentecost as went there for Passover, as travelling conditions were much better in their equivalent date to our June rather than April. This explains where there was such a large contingent of those Jews living abroad and reminds us that due to the Pax Romana there was relatively easy travel within the Empire.
Pentecost celebrated the coming of the harvest and the giving of the Law to Moses at Mount Sinai. The disciples-120 of them- were gathered expectantly in the upper room, waiting for the Holy Spirit's arrival, and when He came everything changed. There were visible manifestations in fire and the effects of the wind, aural effects in the sound of the wind and evidence of the Holy Spirit being poured out on those there by their words. There was a lot of use of tongues in some of the early Church and Paul writes about its use in his letters to the Corinthians and elsewhere and often the utterances were in an angelic language. Here, however, the languages the followers of Jesus spoke were ones from elsewhere and as they burst out of the upper room and began to proclaim praises to God, listeners in the crowded streets picked up on one as being their own.
These men and women, all disciples, who not so long previously had been hiding, dreading the knock on the door from the Jewish authorities, were now loudly telling forth the wonders of God in the middle of Jerusalem at this key moment in the Jewish calendar! The perfect timing of the Holy Spirit enabled God to be glorified in a new way: there was a new Law which was to be written on men and women's hearts rather than being external and there was to be a different sort of harvest, the harvesting of men and women's souls as they turned to receive the saving love of the Lord Jesus Christ!
The prophecies of Joel in Joel 2 v 28, 29, Ezekiel in Ezekiel 37 v 1-11 and others had been fulfilled!