Acts 2:1
When the day of Pentecost came, they were all together in one place — Pentecost (Shavuot), fifty days after Passover, was the Jewish festival celebrating the giving of the Torah at Sinai; the choice of this festival for the Spirit's outpouring connects Jesus' new covenant to God's ancient covenant-making. The unity (homou, together, in one accord) is both literal and theological: the church is born in unanimity and koinonia. The upper room becomes the Sinai of the New Testament, where God's law is written on hearts.