Pentecost is an important Christian feast day that commemorates the descent of the Holy Spirit upon the Apostles and other followers of Jesus Christ while they were in Jerusalem, as described in the Acts of the Apostles (Acts 2:1–31). The event signifies the birth of the Christian Church by the power of the Holy Spirit and is celebrated on the seventh Sunday after Easter, fifty days after Easter Sunday. For this reason, Pentecost is sometimes referred to as “the fiftieth day” or “Whitsunday,” particularly in the United Kingdom.