Day of the Wholy Trinity is one of the biggest Christian holidays. It is celebrated on the fiftieth day after Easter, in 2017 it is June 4th.
This holiday is included to the Twelve Great Feasts of the Orthodox Church, that is to say 12 most important dates after Pascha.
Holy day commemorates the descent of the Holy Spirit upon the Apostles promised by Jesus Christ before the Ascension.
The descent of the Holy Spirit pointed to the Trinity, after which people started to glorify hypostases — the Father, the Son (Jesus Christ), and the Holy Spirit.
Holiday entered the Christian calendar officially only at the end of the fourth century, when the Church adopted the Trinity dogma at the Second Ecumenical Council in Constantinople in 381.