In the Roman Catholic Church it is officially known as the Solemnity of the 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 sixth Sunday in Easter (five weeks 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 primary of the first class by Pope Pius X on 24 July 1911.
The Thursday after Trinity Sunday is observed as the Feast of Corpus Christi, except in the United States and Spain, when it falls on the following Sunday instead. After Corpus Christi, Ordinary Time resumes, and green vestments are worn by the priest and celebrants for the rest of the liturgical year, except on certain special days, including those designated as a holy day of obligation. The first Sunday of Advent marks the beginning of the new liturgical year.
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Trinity Sunday Text: Wikipedia