When the time for Pentecost was fulfilled, they were all in one place together. And suddenly there came from the sky a noise like a strong driving wind,
and it filled the entire house in which they were.  Then there appeared to them tongues as of fire, which parted and came to rest on each one of them.
And they were all filled with the Holy Spirit.  Acts 2:1-3a

What is Pentecost?  Our friends from Catholic Central put this cute short video together.