In the Catholic Church it is officially known as the Solemnity of Wikipediathe Most Holy Trinity. Prior to the reforms of the Second Vatican Council, it marked the end of a three-week period when church weddings were forbidden. The period began on Rogation Sunday, the fifth Sunday after Easter. Trinity Sunday was established as a Double of the Second Class by Pope John XXII to celebrate the Trinity. It was raised to the dignity of a Double of the First Class by Pope Pius X on 24 July 1911.
In the traditional Divine Office, the Athanasian Creed (Quicumque vult) is said on this day at Prime. Before 1960, it was said on all Sundays after Epiphany and Pentecost which do not fall within Octaves or on which a feast of Double rank or higher was celebrated or commemorated, as well as on Trinity Sunday. The 1960 reforms reduced it to once a year, on this Sunday.
In the 1962 Missal, the Mass for the First Sunday After Pentecost is not said or commemorated on Sunday (it is permanently impeded there by Trinity Sunday), but is used during the week if the ferial Mass is being said.
The Thursday after Trinity Sunday is observed as the Feast of Corpus Christi. In some countries, including the United States,Canada, and Spain, it may be celebrated on the following Sunday, when parishioners are more likely to attend Mass and be able to celebrate the feast.