Trinity Sunday is the first Sunday after Pentecost in the Eestern Christian
liturgical calendar, and the Sunday of Pentecost in Eastern Christianity.
Trinity Sunday celebrates the Christian dogma of the Trinity, the three
Persons of God: the Father, the Son, and the Holy Spirit.

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
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
atPrime. 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.  READ MORE