The celebration of Holy Trinity Sunday — the Sunday dedicated to honoring the mystery of the Triune God (Father, Son, and Holy Spirit) — has an interesting development in the history of the Catholic Church. In the early Church (first few centuries), there was no specific feast day solely dedicated to the Holy Trinity. Instead, belief in the Trinity was celebrated implicitly in almost every Mass, because Christian worship was (and is) always directed to the Father, through the Son, in the Holy Spirit. The early Church was more focused on defining the doctrine of the Trinity, especially during controversies like Arianism (which denied the full divinity of Jesus). The Council of Nicaea (325 AD) and later Council of Constantinople (381 AD) formally articulated Trinitarian doctrine.

As heresies continued in the Post-Nicene era and the Church’s doctrine matured, there was a growing desire to have a specific liturgical celebration that explicitly honored the Trinity. By the 8th century, some monasteries, especially in parts of France and Germany, began celebrating a local feast in honor of the Holy Trinity. In the 9th–10th centuries devotion grew, especially promoted by monks and theologians (like the Benedictines).  In the 11th century Pope Alexander II reportedly opposed making it a universal feast, saying that the Church daily honored the Trinity liturgically. However, the idea continued gaining ground in various regions.