It is a Christian holiday which is celebrated on the seventh Sunday after Easter. It commemorates the descent of the Holy Spirit upon the Apostles and other followers of Jesus Christ while they were in Jerusalem celebrating the Feast of Weeks as illustrated in the Acts of the Apostles (Acts 2:1–31). This event represents the birth of the early Church in the tradition of Christian.
Pentecost can also refer to the entire fifty days of Easter through Pentecost inclusive in the Eastern Christianity. So the book comprising the liturgical texts is called the "Pentecostarion". Pentecost is a moveable feast as its date depends on the date of Easter.