"The Holy Spirit" (Acts 1:2,5,8,16) - This was the time for the fulfilment of promises made by Jesus during His earthly ministry (John 14:16,25; John 16:12).
"The Holy Spirit" (Acts 2:4,17-18,33,38) - God is doing a new thing. This far surpasses all that happened in the Old Testament. What had been promised is now reality. From prophecy to power - This is what happened on the Day of Pentecost.