Pentecost is the Greek name for Shavuot, the Feast of Weeks, a prominent feast in the calendar of ancient Israel celebrating the giving of the Law to Moses at Sinai.  In Christianity, Pentecost is celebrated fifty days after Easter Sunday, hence the name.  The feast is also called “Whitsunday.”

In the New Testament, Pentecost was the occasion of the outpouring of the Holy Spirit upon the Apostles and other followers of Jesus, as described in Acts 2 (one of the readings for Sunday), and therefore in the Christian liturgical year, it became a feast commemorating this occasion.  For this reason, it is described by some Christians as the “Birthday of the Church.”