Short answers, pulled from the story.
Trinity Sunday falls on the first Sunday after Pentecost in the Western Christian calendar, eight weeks after Easter Sunday. Its earliest possible date is the 17th of May, as in 1818 and 2285, and its latest possible date is the 20th of June, as in 1943 and 2038.
Trinity Sunday celebrates the Christian doctrine of the Trinity, the belief that God exists as three Persons: the Father, the Son, and the Holy Spirit.
Pope John XXII, who reigned from 1316 to 1334, ordered Trinity Sunday for the entire Church on the first Sunday after Pentecost and ranked it a Double of the Second Class. Pope Pius X later raised it to a primary feast of the first class on the 24th of July 1911.
In the Eastern Orthodox, Eastern Catholic, and Eastern Lutheran churches, Trinity Sunday is the Sunday of Pentecost itself rather than the Sunday after it. The following Sunday is instead called All Saints Sunday, and green vestments are worn for Pentecost and its Afterfeast.
Thomas Becket, who lived from 1118 to 1170, was consecrated Archbishop of Canterbury on the Sunday after Pentecost, known in England as Whitsun. His later martyrdom may have helped popularize the feast in England, from where its observance is said to have spread through western Christendom.
The oldest hymn still sung on Trinity Sunday is the 12th-century Italian hymn Alta Trinità beata, recorded in the Laudario di Cortona. Johann Sebastian Bach also composed cantatas for the day, including three that survive today: BWV 165, BWV 176, and BWV 129.