The Holy Spirit did not make His first appearance at Pentecost. He is mentioned as early as the creation account. Including throughout the Old Testament and gospels. The Spirit arrived in the upper room with a fresh mission from the Father.
Jesus’ followers were gathered together for Pentecost, and the Holy Spirit “filled the whole house where they were sitting” (Acts 2:2). “All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them” (Acts 2:4).