Reference

John 20:19-23
The Festival

Pentecost had been a festival for millennia by the time those followers of Jesus were gathered in that room, 50 days after the Resurrection and 10 days after Jesus ascended to heaven.

Pentecost was a kind of combination harvest celebration and beer fest.  Yes, beer fest.

But when the Holy Spirit showed up, Pentecost became a party.  And everyone is invited.