After the Pentecost celebration, the Rite of the Catholic Church further meditates on the great mystery of God through the feast of Trinity Sunday.
Because of that, the Sunday following Pentecost has been dedicated to the Holy Trinity for many centuries, though it was not always celebrated by everyone in the Church.
In the Catholic Encyclopedia, at the first time this Sunday was known as a Dominica vacans, with no specific focus or theme.
At that period, there was no particular feast to celebrate the Holy Trinity, but later on there arose a need for the Church to further define her beliefs in God.
During the 4th Century, a Heresy known as the Arian heresy began to spread and disputing the traditional Christian belief of one God in three divine persons.
Due to that fact, the bishops of the Church decided to compose a Mass in honor of the Trinity in order to reaffirm the belief, but this was not given a specific date in the calendar.
However, as at 8th and 9th century, the Church found a perfect day. The St. Andrew Daily Missal helps to explain better on how Sunday was the most fitting day.
Sunday is considered to the feast of Trinity because, it was consecrated throughout the year to the Holy Trinity because God the Father began the work of creation on the “first day,” God the Son made man rose from the dead on a Sunday morning, and God the Holy Spirit came down on the Apostles on Pentecost Sunday.
Aside from celebrating the Trinity in some fashion on each Sunday, there is also a need to fill the “vacant” Sunday after the feast of Pentecost. This need was arose by the fact that ordinations happened during this time and there existed no specific liturgy.
Furthermore, the St. Andrew Daily Missal explains, “The feast of the Holy Trinity traces its origin to the fact that the ordinations of the Ember Saturday, which took place in the evening, were prolonged to the next day, which was Sunday and had no proper Mass at that date … [a votive Mass of the Holy Trinity] was celebrated in some places on this Sunday; and since it occupied a fixed place in the liturgical calendar, this Mass was considered as establishing this Sunday as a special feast of the Blessed Trinity.”
Moreover, celebrating Trinity Sunday after Pentecost also allows the Church to reflect further on the mystery of God after receiving the gift of the Holy Spirit.
This however, helps us to see the connection that if we truly want to understand the Trinity, we really need to have the gift of the Holy Spirit.
Left for us, we can never fully understand who God is on our own and so, we desperately need his guidance and inspiration.
Above all, the Trinity is one of the most fundamental beliefs of the Catholic Church and so it is fitting that we dedicate a particular Sunday to that mystery.