Trinity Sunday falls on the first Sunday after Pentecost in the Catholic liturgical calendar (the Sunday of Pentecost in Eastern Orthodoxy) and celebrates the doctrine of the Trinity.
Prior to the Second Vatican Council, the Solemnity of the Most Holy Trinity marked the end of the three week period that began on Rogation Sunday. Weddings were traditionally not allowed during this period. Pope John XXII established the feast as a Double of the Second Class to celebrate the Trinity and was raised to a Double of the First Class on July 24th, 1911. In the traditional Divine Office, the Athanasian Creed is said on this day at Prime.
Historically, the early Church originally held no special Office or day for the Holy Trinity. Pope Alexander II refused a petition for the special feast on the criteria that by it's very nature the Holy Catholic Church honored the Trinity daily. Pope John XXII ordered the feast for the entire Church as the Sunday following Pentecost.
The Thursday following Holy Trinity Sunday is observed as the Feast of Corpus Christi though many countries celebrate the feast day on the following Sunday.