In Christianity, Pentecost (Day of the Holy Trinity) is celebrated fifty days after Easter Sunday, inclusively (i.e., 49 days with the first day counted, seven weeks), hence its name. Pentecost falls on the tenth day after Ascension Thursday (which itself is 40 days after Easter).
The feast is also called White Sunday, or "Whitsunday", especially in the United Kingdom, where traditionally the next day, Whit Monday, was also a public holiday. In Eastern Christianity, Pentecost can also refer to the entire fifty days between Easter and Pentecost, hence the book containing the liturgical texts for Paschaltide is called the Pentecostarion.
In the New Testament, Pentecost was the occasion of the descent of the Holy Spirit upon the Apostles and other followers of Jesus Christ, as described in the Acts of the Apostles 2:1–31. and therefore in the Christian liturgical year, it became a feast commemorating this occasion. For this reason, Pentecost is described by some Christians as the "Birthday of the Church". The Pentecostal movement of Christianity derives its name from this New Testament event, as the movement emphasizes direct personal experience with God, akin to the Descent of the Holy Spirit upon the Apostles.
Source: wikipedia.org | The Holy Bible
In 2016 Day of the Holy Trinity. Pentecost in USA falls on May 15.