When the day of Pentecost arrived, they were all together in one place.
Pentecost was a popular day in the old Jewish calendar. The name comes from the fact that it was held 50 days after the great Festival of Passover (the Greek word for five is "Pente"). It was a happy festival because it also marked the end of bringing in the harvests. It was a festival for which all Jewish men were expected to go to Jerusalem, to celebrate there. We are told that the city was full of people from all over those areas of the world where Jewish people had settled — mostly areas surrounding the Mediterranean Sea.
For Christians, this day became specially important because it was on this day that God the Holy Spirit was "poured out" from heaven like "a mighty rushing wind", and "divided tongues as of fire" came upon all the people who were gathered together waiting, as Jesus told them to do, for just such an event as this (v2-3).
What did it all mean? First of all it meant Jesus had kept his promise to send the Holy Spirit. Secondly, it gave new courage, power and authority to the apostles and other followers of Jesus. In the third place the great miracle of the apostles speaking in the many languages of the people who were from many different countries meant they could understand the Gospel and tell people about it when they went home. And fourthly this was like the 'birthday' of the Christian Church as it began to go into all the world.
Dear Lord Jesus, thank you for the Holy Spirit and his place in my life. Amen.