The Solemnity of the Most Holy Trinity: First Sunday After Pentecost
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. In the early 4th century when the Arian heresy was spreading, the early church, recognizing the inherent Christological and Trinitarian implications, prepared an Office of Prayer with canticles, responses, a preface, and hymns, to be recited on Sundays to proclaim the Holy Trinity. Pope John XXII (14th century) instituted the celebration as a feast for the entire Church; the celebration became a solemnity after the liturgical reforms of Vatican II.
While not an exclusive Trinitarian reference, the Trinity can be seen in the “Mary Being Crowned Queen of Heaven” window above the central entrance to the church, featuring Jesus to the left, God to the right, and the Holy Spirit as a dove central and above them.