Trinity Sunday is the first Sunday after Pentecost in the Western Christian liturgical calendar, however, in Eastern Christianity there is no specific day set aside to celebrate the Christian doctrine of the Trinity. The Eastern Churches point out that they celebrate the Trinity every Sunday. Westerners do as well, we just set aside a special feast day for the purpose. In the West, Trinity Sunday, officially called “The Solemnity of the Most Holy Trinity,” is one of the few celebrations of the Christian Year that commemorates a reality and doctrine rather than an event or person.
Holy Trinity Sunday is celebrated on the first Sunday following Pentecost in most of the liturgical churches in Western Christianity. It is a solemn celebration of the belief in the revelation of one God, yet three divine persons. It was not uniquely celebrated in the early church, but as with many things the advent of new, sometimes heretical, thinking often gives the Church a moment in which to explain and celebrate its own traditions; things it already believes and holds dear.