50 days after Easter Sunday - the day on which Jesus rose from the dead –
all the Jews were gathered to celebrate the Feast of Weeks (Pentecost)
Acts 2:1 (NIV 1984)
When the day of Pentecost came, they were all together in one place.
The disciples were also gathered in the Upper Room.
And what happens next is prophesied.
Acts 2:2-4 (NIV1984)
Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.