In the beginning of Acts 2, the disciples had returned to Jerusalem from Mt. They had seen the ascension of Jesus, but they weren't sad, as some might have expected. Jesus had promised to send a Comforter, and they could hardly wait. They were excited and were gathered in the temple, praising God and waiting. Luke 24: 53 reads that they were "continually in the temple, praising and blessing God." They remembered the words that Jesus said, "Whatsoever ye shall ask the Father in My name, He will give it to you. Hitherto have ye asked nothing in My name. Ask, and ye shall receive, that your joy may be full" (John 16:23). The disciples believed this promise, truly sought God, humbled their hearts with united minds and spirit, and waited on the Holy Ghost to fall. .
Jesus not only promised to send a Comforter, but power as well. He said in Luke 24:49, "And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem until ye be endued with power from on high." The Holy Ghost infilling brings not only comfort and peace, but power also.
On the day of Pentecost, people were gathered in Jerusalem from miles around to celebrate. The feast of Pentecost, first day of the week, was kept in remembrance of the law on Mt. Sinai where the Jewish church got its start. Because of the masses gathered, this was a perfect day for the Holy Ghost to come down. There would be hundreds of witnesses and the word of the gospel and the awesome events could be spread quickly and more easily. .
In the Old Testament, when God came down He presented Himself in a cloud. When Jesus ascended back to heaven, He did this in a cloud. But in Acts 2, the Holy Ghost was quite different. "Suddenly there was a sound from heaven like a mighty rushing wind." Being completely unprepared, the disciples were probably startled, but more importantly their undivided attention was gained. The sound not only filled the one room, but the whole house, the streets, and maybe even houses beside them.