Trinity Sunday, also known as the Feast of the Holy Trinity, is observed on the first Sunday after Pentecost. On this feast day, Christians celebrate the Holy Trinity which consists of God, the Father; Jesus, the son; and the Holy Spirit.
Trinity Sunday is not a federal holiday in the United States.
Records of the use of the Trinitarian baptismal formula during early liturgies suggest that churches had been celebrating the Holy Trinity for centuries before Trinity Sunday was officially established as an observance on 828 CE by Pope Gregory IX. This was an unusual holiday for the Christian church, as it celebrates a doctrine rather than an event in the history of the religion. Also, at the time, the church was divided on the idea of God being three different elements in one - meaning the Father, the Son, and the Holy Spirit-, and so, it wasn't until 1334 that Pope John XXII declared it to be a feast day, and Trinity Sunday began to be observed by Christians around the world.
Customs and Traditions
On Trinity Sunday the liturgy during mass is aimed at honoring the Holy Trinity of the Father, the Son, and the Holy Spirit. Churches will be decorated with the liturgical color for Trinity Sunday, which is white, and often they will feature symbols of the Holy Trinity. This could be the more common triangle made of overlapping oval loops, or other representations of the Trinity which are often a crown for the Father, a lamb to represent Jesus, and the symbol of the dove is widely known to represent the Holy Spirit.
During mass, churches will conduct special prayers for Trinity Sunday. The most commonly known one is the Athanasian Creed, which heavily features the affirmation of God as a triune. After the Sunday service, many churches organize meals for the congregation to share, and it is customary for there to be choral concerts during Trinity Sunday.