As those in the room went out among the crowd to spread the Good News, some accused those speaking in tongues of being intoxicated. Peter explained to the crowd that this was the work of the Holy Spirit, as prophesied in Scripture, and began preaching to them. Peter then called on those present – about 3,000 people – to be baptized. That moment firmly established Peter as the leader of the faith – the “rock” Jesus had previously told him he would become – and is known as the birthday of the Church.
The timing of these feasts is where the concept of the novena – nine days of prayer – originated. Acts 1 says that Mary and the apostles prayed together “continuously” for nine days after the Ascension, leading up to Pentecost. Traditionally, the Church prays the novena to the Holy Spirit in the days before Pentecost. Look for that and other novenas online.