Pentecost means “fiftieth” in Greek. Pentecost happened exactly fifty days after Easter. It marks the official end of the Easter season.
At Pentecost we celebrate when the Holy Spirit visited Mary, the apostles, and the other followers of Jesus. Jesus had ascended to heaven nine days earlier (this is why we pray a novena for nine days!), and since then, Mary and the other apostles had been praying continuously. On Pentecost, the Holy Spirit came like a strong, driving wind into the room where they were praying. Soon, fire rested on their heads and gave them the gift of being able to speak in different languages and still understand each other. The Holy Spirit also gave the apostles the other gifts they would need to go out and spread the Gospel.