We celebrate Pentecost this Sunday. The day of Pentecost came ten days after Jesus' ascension into heaven and fifty days after his resurrection, during the Jewish festival known as the Feast of Weeks, when people from many nations returned to Jerusalem. It is connected to and fulfills the worship of the Old Testament, when God commanded his people to celebrate the day after seven weeks after the Passover (the fiftieth day) as a Sabbath day. The number fifty is a sign of completeness and fulfillment in the Bible, and is also connected to theme of the jubilee year when debts were forgiven and slaves were freed.
After Jesus' ascension into heaven, to take his throne at the right hand of God, he sends his Spirit as promised to his church on earth. If the Spirit did not descend and remain with us, we would have been left alone, so we celebrate Pentecost as the fulfillment of Jesus' promise not to leave us as orphans. Pentecost is also our celebration of the birth of the church. It is the work of the Spirit in transforming lives that causes the church to grow and spread from Pentecost on. The season after Pentecost, called Ordinary Time, is the longest season of the year, where the church is reminded of the work of the Spirit in our daily lives, inaugurating the kingdom of God here on earth.