This is a Christian holiday celebrated on the seventh Sunday after Easter. It commemorates the Holy Spirit's descent upon the Apostles and other disciples 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 Christian tradition.
In Eastern Christianity, Pentecost can also refer to the entire fifty days of Easter where Pentecost is included. Thus, the book containing the liturgical texts is called "Pentecostarion". Pentecost is a moving holiday as its date depends on the date of Easter.