The Christian holiday of Pentecost is a moveable feast, which is celebrated on the 50th day (the seventh Sunday) from Easter Sunday. 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 described in the Acts of the Apostles (Acts 2:1–31).
The holiday is also called “White Sunday” or “Whitsunday” or “Whitsun”, especially in the United Kingdom, where traditionally the next day, Whit Monday, was also a public holiday (since 1971 fixed by statute on the last Monday in May). The Monday after Pentecost is a legal holiday in many European countries.
In Eastern Christianity, Pentecost can also refer to the entire fifty days of Easter through Pentecost inclusive; hence the book containing the liturgical texts is called the “Pentecostarion“. Since its date depends on the date of Easter, Pentecost is a “moveable feast“.
Pentecost is one of the Great feasts in the Eastern Orthodox Church, a Solemnity in the Roman Rite of the Catholic Church, a Festival in the Lutheran Churches, and a Principal Feast in the Anglican Communion. Many Christian denominations provide a special liturgy for this holy celebration. (Source)