"Pentecost marks when the Holy Spirit descended upon the Apostles of Jesus, Mary, and other followers of Christ, 50 days after Easter — hence the name, which comes from a Hellenistic Greek word meaning "50th." The followers of Christ had gathered in Jerusalem to observe the Feast of Weeks (Shavuot in Hebrew), the harvest festival held seven weeks and one day after the first day of Passover.
"The disciples were lost after the death of Jesus. They didn't know what to do next. Then the Holy Spirit came upon them, and they felt its vitality and strength. They went out to the streets and started preaching. They inspired 3,000 people to become baptized." From DW
I had to reach 80 to learn what they mean by the Holy Spirit. Now it makes sense.