Trinity Sunday is the first Sunday after Pentecost in the Christian liturgical calendar.

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 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 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, 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, until Advent begins, except on certain special days, including those designated as a holy day of obligation.

Other times when church weddings were forbidden included:

• The first Sunday of Advent through January 13, the end of the Octave of Epiphany.
• Septuagesima through Low Sunday (the first Sunday after Easter).